Apparently this is caused by the default MenuItem style, which wasn't adapted yet, as I only redefined the ControlTemplates for the different types of MenuItems.

I got rid of these messages by adding this to my application-wide styles:

    <Style TargetType="{x:Type MenuItem}">
        <Setter Property="HorizontalContentAlignment" Value="Left"/>
        <Setter Property="VerticalContentAlignment" Value="Center"/>
    </Style>