System.Runtime vs mscorlib in a CompositeFont - wpf

Question

We are in the process of migrating our WPF application from .NET Framework to .NET Core 3.1.
While working on this, I had to exchange all references in xaml from xmlns:system="clr-namespace:System;assembly=mscorlib" to xmlns:system="clr-namespace:System;assembly=System.Runtime". Makes perfect sense, mscorlib is for .NET Framework and System.Runtime is for .NET Core. Read more here.
Starting up the application I found out there was an issue with a composite font not being loaded. Because I just replace mscorlib with System.Runtime, the file would look like this:
<FontFamily xmlns="http://schemas.microsoft.com/winfx/2006/xaml/composite-font"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:system="clr-namespace:System;assembly=System.Runtime">
<!-- Name mapping -->
<FontFamily.FamilyNames>
<system:String x:Key="en-US">My Font</system:String>
</FontFamily.FamilyNames>
<FontFamily.FamilyMaps>
...
</FontFamily.FamilyMaps>
</FontFamily>
But I still had problems. So I changed this one file back to use mscorlib like this:
<FontFamily xmlns="http://schemas.microsoft.com/winfx/2006/xaml/composite-font"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:system="clr-namespace:System;assembly=mscorlib">
<!-- Name mapping -->
<FontFamily.FamilyNames>
<system:String x:Key="en-US">Cabo Font</system:String>
</FontFamily.FamilyNames>
<FontFamily.FamilyMaps>
...
</FontFamily.FamilyMaps>
</FontFamily>
And now everything works perfect.
Anyone with a good explanation of why this is?

Answer 1

While working on this, I had to exchange all references in xaml from xmlns:system="clr-namespace:System;assembly=mscorlib" to xmlns:system="clr-namespace:System;assembly=System.Runtime"
You shouldn't have to do this since any reference to mscorlib is type forwarded to System.Runtime automatically: https://github.com/dotnet/wpf/issues/708.
This means that the very same markup that you used when targeting the .NET Framework should also work when you target .NET Core.