Draft: Fix flag delete method

This is probably a bit more difficult than what I wanted to do in !69763 (merged), so I'm breaking it off.

I think I might go back to just forbidding deletion for flags that have been cloned. This would mean adding some kind of flag that indicates this to the cloneAndReplace and clone methods.

I might also add some unit tests for the iterators, which (I think) might also be broken.