Custom Reader/Writer not active for NetworkMessages, but for SyncVars

[Kerozard]

Describe the bug
When adding a custom reader/writer for an abstract class, it works for synchronizing SyncVars, but when adding the abstract class as a field to a network message, an error message is produced after a recompile.

The error message: Error: (0,0): error Cannot generate read function for abstract class Foo. Use a supported type or provide a custom read function

Repro project

[NetworkMessage]
public struct FooMessage
{
  public Foo Foo;
}

public abstract class Foo {}
public class FooBar : Foo {}

public static class FooReaderWriter
{
  public static Foo ReadFoo(this NetworkReader reader)
  {
    var fooValue = reader.ReadInt32();
    return new FooBar();
  }

  public static void WriteFoo(this NetworkWriter writer, Foo foo)
  {
    writer.WriteInt32(0);
  }
}

To Reproduce
Steps to reproduce the behavior:

1. Copy the attached code into a new cs file inside the project
2. Compile
3. See error

Expected behavior
Custom reader/writer should also work correctly for network messages.

Desktop (please complete the following information):

• OS: Windows 10 Pro
• Build target: PC, Mac & Linux Standalone
• Unity version: 2020.3.16f1 (LTS)
• Mirage version: 102.0.0