@Vincent 넵 맞습니다.
TabItem에 TabStripPlacement (Left, Right, Top, Bottom) 관련해서 MultiTrigger가 RelativeSource AncestorType 으로 수십 개 정의되어 있어요.
그래서 삭제하기 전에 Template을 제거하시면 문제 없었던 거죠. 
Template을 새롭게 만드시면 됩니다!
보통 TabItem Template을 커스터마이징 하니까 자연스럽게 해결되는 문제 같아요.
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Left"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Border}"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,0,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,0,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Bottom"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Border}"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,0,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,0,1,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Right"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Border}"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="0,1,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="0,1,1,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Top"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.MouseOver.Border}"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,1,0"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,1,0"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Left"/>
</MultiDataTrigger.Conditions>
<Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Border}"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,0,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,0,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Bottom"/>
</MultiDataTrigger.Conditions>
<Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Border}"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,0,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,0,1,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Right"/>
</MultiDataTrigger.Conditions>
<Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Border}"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="0,1,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="0,1,1,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsEnabled, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Top"/>
</MultiDataTrigger.Conditions>
<Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
<Setter Property="Background" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Background}"/>
<Setter Property="BorderBrush" TargetName="mainBorder" Value="{StaticResource TabItem.Disabled.Border}"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,1,0"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,1,0"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Left"/>
</MultiDataTrigger.Conditions>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,0,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,0,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Left"/>
</MultiDataTrigger.Conditions>
<Setter Property="Panel.ZIndex" Value="1"/>
<Setter Property="Margin" Value="-2,-2,0,-2"/>
<Setter Property="Opacity" TargetName="innerBorder" Value="1"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,0,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,0,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Bottom"/>
</MultiDataTrigger.Conditions>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,0,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,0,1,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Bottom"/>
</MultiDataTrigger.Conditions>
<Setter Property="Panel.ZIndex" Value="1"/>
<Setter Property="Margin" Value="-2,0,-2,-2"/>
<Setter Property="Opacity" TargetName="innerBorder" Value="1"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,0,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,0,1,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Right"/>
</MultiDataTrigger.Conditions>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="0,1,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="0,1,1,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Right"/>
</MultiDataTrigger.Conditions>
<Setter Property="Panel.ZIndex" Value="1"/>
<Setter Property="Margin" Value="0,-2,-2,-2"/>
<Setter Property="Opacity" TargetName="innerBorder" Value="1"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="0,1,1,1"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="0,1,1,1"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="false"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Top"/>
</MultiDataTrigger.Conditions>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,1,0"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,1,0"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="true"/>
<Condition Binding="{Binding TabStripPlacement, RelativeSource={RelativeSource AncestorType={x:Type TabControl}}}" Value="Top"/>
</MultiDataTrigger.Conditions>
<Setter Property="Panel.ZIndex" Value="1"/>
<Setter Property="Margin" Value="-2,-2,-2,0"/>
<Setter Property="Opacity" TargetName="innerBorder" Value="1"/>
<Setter Property="BorderThickness" TargetName="innerBorder" Value="1,1,1,0"/>
<Setter Property="BorderThickness" TargetName="mainBorder" Value="1,1,1,0"/>
</MultiDataTrigger>