Resetting it didn't fix it, but possibly because I restored a backup.

Might be a bug Apple added to the latest firmware. Or the radio is failing in it. If it doesn't get fixed by the next update, I think I'll sell the watch as is, and get a non Apple Watch. Selling is a pain, so maybe just get a different watch.

I'm too lazy to try and figure out how to report bugs to Apple. Somebody said that's a full time job.

Somebody else said on a forum, that the Apple watch will disconnect from WiFi and use Bluetooth instead, if it's in range of Bluetooth. Pretty sure that's false, as rebooting connects to WiFi, and stays connected. And it was staying connected just fine, before the update. Maybe that's the case with older watches. Shouldn't stay connected to WiFi after rebooting, if that's the case.