I have recently discovered that if a CKRecord is saved with a new parent reference, the parent's change tag might be changed by CloudKit behind the scenes. The client is not notified about this change in any way.
If you rely on the change tag to ensure consistency between your local and remote records, you might have unexpected problems once you start using sharing in CloudKit.
I reported that to Apple (bug report 27396718) and at first they replied that this is an intended behaviour and closed the report. Fortunately, after I provided some arguments to them they re-opened the report and promised to see if there is anything that they can do about it.
If you believe that this issue should be fixed, please create a new bug report at bugreport.apple.com mentioning 27396718 as the original report. Apple is using duplicate reports as a "voting system" to identify issues that should be prioritized. This is the only way for us (developers) to ensure that this gets fixed before it is not too late in terms of iOS 10 development cycle.