Like backups, Heath data should sync automatically when your watch is paired to your phone and they both can connect to wifi.
I would take the watch and phone to a different wifi and possibly reset network settings to ensure they both can join wifi. Some watches have different chips so you may need Apple support to help you determine which wifi channels both devices support.
If that doesn’t work, I would unpair the watch to force a backup and sync and then setup the watch again.
Something is amiss and these are the items I’ve seen people get stuck on and one I had to do once quite a few years back. Is this the same hardware you needed to work on previously?