I have the same problem with AW5 stuck on 100% and battery errors.
Bought first in October and within a few days battery stuck on 100%.
Brought it to Apple store and the test says battery was OK so they offered to send it off for repair.
Since I had just bought it I said I will just return it.
So they replaced it with a new AW5.
Yesterday the same problem recurred.
Stuck on 100% and then just shuts off.
This morning this started getting weirder - ran down to 77% then shut off.
Put it on charger and when it came alive showed 13% and it charged to 99% in 30 minutes...thats not right.
Contacted Apple support chat and they had nothing to offer but the usual.
So off to the Apple store tomorrow ....again.
I will ask for a refund...don't want to try a third watch...jamais deux sans trois.
And yes I tried all the fixes...many times and nothing work.