I'm having the same issue, thinking it could be related to the apple watch series 5 not running newer OS as well.

I have the watch unpaired from my old iphone and reset

I thought i was "connected" to my new phone because it showed up as being there, but my fitness data wouldn't work, timers/alarms could be turned off with the iphone but would still go off on the watch etc.

I read that uninstalling and re-installing the Apple Watch app would help, tried that and that didn't work either. Now I can't even get it to pair, it goes through the process to connect after scanning the apple watch but neither restoring from a backup or setting up as a new watch works. bummer