It seems to me that the configurations (filter, sort, etc.) and/or field names in the data from your UAT base might've been adjusted after you did the initial sync with the Prod base? I've noticed with the sync, some changes you make to the original sync source (after two bases are synced together) don't carry over and you need to make the same changes in the other base (Prod base in your case). It's not a problem of the data copying over properly, it's the configurations/structure of your data that don't carry over via sync. LMK if that helps.
P.S
Custom builds for your needs. Reach out if you want to talk!