Thanks for the reply!
Firstly, correct, we're using CoreData + CloudKit and "mirrored relationships" means "reflexive relationships" (e.g., Article.similarArticles whereby both are of the type Article).
Assuming that you are using Data + CloudKit, I am guessing that this can be triggered by your code and data.
Regrettably, that doesn't seem to be the case, also given the Delete Rule is "Nullify". Digging through our old messages, we found out that we had already reported this very same issue a couple of years ago with a similar data model but where no entity was ever deleted. At that time we filed feedback report 9118745 and we were told that this was likely a bug.
it is worth filing a feedback report for the Core Data team
Given this seems to be the very same issue, I’ve added the information of this post to our above ticket (and have provided the relevant system diagnose logs, data model, etc. there too).
Does your Core Data model indeed has the reflexive relationship? If yes, has it been there for long time, or did you add that recently before you saw the error?
Yes, we've added this reflexive relationship to a completely new project/iCloud container in 2024 and the reflexive relationship indeed synced, however, not in all instances. So it looks like NSPersistentCloudKitContainer indeed supports reflexive relationships, but is somehow confused and fails only in certain instances.
Many thanks