I was able to find this question after some further digging: https://developer.apple.com/forums/thread/654392?answerId=621352022#621352022 And it helped identify the source of the problem - that I had effectively created a loop with my .deleteSelf actions I was creating on various references. Once I double checked and adjusted those, the error went away and I was able to save. Spotting this was made more complex by the fact that I wasn't changing the .deleteSelf actions version what I had previously done - and was working fine - when my saves were done in sequence rather than a bulk CKModifyRecordsOperation save.

Issue for me for over 12 hours....