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.
kesterDL
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
From
Seattle, WA, United States
Post
Replies
Boosts
Views
Activity
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.