I have a VivioActive 3 Music.
I had the same problem as these other users:
I contacted Garmin support via chat about 2 weeks ago. They told me I had to buy a new watch.
Tonight I researched more & found these others users who had the same problem and their solution.
Their thread was locked so I couldn’t add my comment there.
My WHR sensor & activity/movement sensor are both working again after following the reset instructions in the above thread.
It is worth mentioning that I had already done a factory reset which apparently doesn’t fix it. But their method did.
Finally after about 1 month my watch is functional again.
In the future I would appreciate proactive notice from Garmin on issues like this after they discover them, so I don’t have to go out and spend hundreds on a new watch because of their own bug.
Fingers crossed that the issues don’t return.