This issue sounds similar to this one here https://developer.apple.com/forums/thread/125363. The solution on this thread was to sign out of iCloud on the device and sign back in. Seems like it may be related to updating the app via TestFlight. I was experiencing the same behavior with my app, and the sign out/sign in worked for me.

Derrick your comment saved my sanity. I also experience the behavior you described. My app stops syncing with iCloud after my app is updated via TestFlight. Signing out of my profile on the device and signing back in fixed the issue.