Can UserControl.Resources reference a different assembly than the control's contents?

Question

I'm having a bizarre problem with one of my UserControls where if I only make use of a xml namespace in the UserControl.Resources the assembly isn't found. But if I also make use of the same xml namespace in the contents of the control then both work properly.

<UserControl x:Class="MyControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:myAssemblyAlias="clr-namespace:MyAssembly;assembly=MyAssembly">
    <UserControl.Resources>
        <ControlTemplate x:Key="AControlTemplate">
            <... other stuff .../>
            <myAssemblyAlias:MyObject/>
        </ControlTemplate>
    </UserControl.Resources>
    <Grid>
        <myAssemblyAlias:MyObject x:Name="MyObjectInContent" />
    </Grid>
</UserControl>

If both instances of MyObject are present it works correctly. If I comment out the line for MyObjectInContent I get a runtime error telling me MyAssembly can't be found.

Anyone ever seen anything like this before? It almost seems like Visual Studio has the reference available properly at compilation but not at runtime.