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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. :)
 
Called Apple twice today with no help given other than to wait 24 hours. I call BS, I left it running plugged in on wi-fi all last night with no change in state. I have erased and then restored to a back up made right before erasing it and iMessages shows the same messages with all the gaps and with things being out of order. My messages are all correct on my iPad and MacBook Pro. This is so frustrating...

Yeah my MacBook Pro and iPad Pro have everything correct but my iPhone is the one with the holes in my text messaging history
 
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