Reported by Rémi Forax in valhalla-dev:

I've just tested several inner classes cases and everything seems fine,
[...]
I've still found a bug
the field that reference the outer class is not marked as ACC_FLATENABLE even if the outer class is a value type.

in this example,
public __ByValue class Outer {
  private final int value;
  
  public Outer(int value) {
    this.value = value;
  }
  
  public __ByValue class Inner {
    private final int value2;
    
    public Inner(int value2) {
      this.value2 = value2;
    }
  }
}

javap says that inside Outer$Inner.class, this$0 is not ACC_FLATENABLE
  final fr.umlv.valuetype.Outer this$0;
    descriptor: Lfr/umlv/valuetype/Outer;
    flags: (0x1010) ACC_FINAL, ACC_SYNTHETIC