I'm running into the same issue now. It was working great with .clipped().compositingGroup() previously. It's in a part of the app that I don't regularly use, so I didn't notice until today that this broke with iOS15.1. I'm also on Xcode 13.2.1, same broken behaviour in the simulator and on a real device.

After struggling with this issue for a day, I find the answer half an hour after posting. The solution is to replace the Form in SubForm with List, and it gives me exactly what I'm looking for.