what steps did you take to fix the issue?
I backed up the XS again to iCloud (to retain all my settings and stuff), erased the phone, then set it up again, choosing to restore from the backup I'd just created. This re-triggers the sync of messages, mail, health data, etc. About 30-45 minutes later everything synced back perfectly.
I set up my phone last Friday, launch day. My completely unscientific guess is that everyone setting up their new phones put a big strain on Apple's servers, which may have caused the inconsistent syncing people have been experiencing. I've got nothing to back that up though, just my two cents.