Have logged FB14083137 for the error that I am seeing. Same code that fails in iOS18 works fine on iOS17.

I see this when updating relationships after upserting a model object in a context. If the model object already exists and the upsert is an update the relationship updates fail with the error. If the model object does not already exist and the upsert is an insert then the relationships update fine. This looks like a bug and I cannot see any way around it.

Thanks! Filed as FB13688595.

Same problem here with beta 3 2023-07-07, followed XSalvador's advice (thanks!) which worked.

Thanks bug is fixed in XCODE Beta 5.