I specifically signed up to comment on this thread as this issue has been a major pain in rear. I think this might be solved for me after spending about an hour with Apple in this. I previously had and Apple iPhone X, then moved to an XS Max. I don't recall if I ever unpaired the watch before resetting that the factory. However, when I got the new phone I paired to the watch no problem. This issue happened. I followed all kinds of suggestions, up to resetting the watch to factory and setting it up as a new watch (no restore from backup) to the new phone...still, the issue happened.
- First thing (I did not have this problem, but support suggested it) - Check to make sure you didn't block your own phone number. Some people do this after receiving spoof phone calls from their own number.
- Second thing (This seems to have fixed it for me) - On your phone, go to Settings -> General -> Reset and "Reset All Settings". This will not erase your account setup (you will need to setup passcode and faceid again) or text messages,emails or apps, photos, etc., but it will erase settings pertaining to WiFi, your background customization, and access for apps to cellular data.
I've only been messing with it for a couple hours, but have sent texts back and forth with people on sms texts and iMessage going both ways and checking my phone has marked it read on my watch and vice versa. I hope this helps. This is an ongoing issue and I don't think I've actually seen it figured out yet. I hope this works for everyone. Please reply if it solved it for you so that we can help other annoyed souls get their watch back in sync.