I'll second that.
Just tried something similar myself.
When running in the simulator, records are definitely saved up to iCloud as I can see them in the CloudKit dashboard (once you update the index to make the recordName queryable). I've tried this from both the simulator and via a second (physical) device.
Whilst I know that notifications don't work on the simulator it doesn't appear that notifications are being received on the physical device either and I'm getting the same error message as you reported in the console for that device.
Running Xcode 11, macOS Catalina. Testing Devices: Simulator / 9.7 iPad Pro iOS 13 Beta.
I'm in the middle of investigating so I'll post anything else I find.
Have you found anything else this far? I get the same error code, although sometimes it works.
I feels as if making changes too frequently to the core data model (creating too many requests?) might lead to this error.
I suspect it's just because it's still very much in the beta phase.
I have an app with about 3600 core data managed objects and they appear to be uploading OK (although it's hard to really know without any sort of % progress feedback). I did get a lot of "Limit Exceeded" errors (syncing more than 400 CKRecords at a time isn't allowed) but it seemed to automatically divide the requests and reschedule them.
However, on my second device, only a fraction of the data has come down and there is no way to tell what percentage has completed so it's anyone's guess as to whether it is 'done' or not...
So far, I'm very impressed that it works at all given that it required effectively no code (especially all that complicated error handling), but I think they still have a way to go before I'm confident enough to roll it out to my users.