I purchased my Series 5 watch about 4 months ago. After 3-3.5 months I started having this issue. Watch would show 70%, I'd go workout and 10 minutes in the watch would die and not restart until I put it on a charger. As soon as it hit the charger it would show 50% battery remaining. That night I saw an update was pending for 6.2.1 and installed it that night and assumed the issue was resolved, but it persisted.
It eventually got to a point where it continually showed 100%, even after being off the charger for 24+ hours.
After 2 hours otp with support doing remote diagnostics, unpairing, factory resetting, etc... they put in the repair ticket and issued a replacement. My new Series 5 should be here tomorrow.
Hopefully this new one lasts more than 3.5 months.