This is on Android with a Pixel 4a.
I had the same problem with my Fenix 5.
It started sometime in June.
When it happens you have to pair the watch with the mobile app again, basically the same you did when you bought the watch.
I don't know if this is Garmin's fault or Google's. I also really don't care. This is a known problem and I just bought a not cheap watch and don't want to deal with this.
Someone sort this out please.