Interface implementation returning inner class isn't correctly patched

[RunninglVlan]

Interface and class in the same file. Method returns inner class.

public interface ITest {
  Test.Builder Create(Entity entity, string prefixId);
}

public class Test : ITest {
  Builder ITest.Create(Entity entity, string prefixId) {
    // ...
    return new Builder();
  }

  public class Builder { }
}

FSR produces:

public interface ITest__Patched_{
  Test.Builder Create(Entity entity, string prefixId);
}

public class Test__Patched_: ITest { // error CS0535
  Builder ITest.Create(Entity entity, string prefixId) { // error CS0539
    // ...
    return new Builder();
  }

  public class Builder { }
}

error CS0535: 'Test__Patched_' does not implement interface member 'ITest.Create(Entity, string)' error CS0539: 'Test__Patched_.Create(Entity, string)' in explicit interface declaration is not found among members of the interface that can be implemented

Moving interface out of the file doesn't help. Same CS0539 and CS0535 errors.