UPDATE: I have been on WatchOS 6.1.2 beta 2 for a few days, and the problem is back with a vengeance. There is a recent 4-page thread on the general Apple Forums, where I posted this:
Stays at 100% all day, the goes to the red lightning bolt without warning. The past day or two, it is dying after a few minutes off the charger, while on vacation. Bummer. Have now done multiple hard resets and unpair/repair. Deleted all non-Apple apps. Turned off Noise, reduced motion. Yet to do: Leaving off for Hour(s)/Days and Freezer.
I am on WatchOS 6.1.2 beta 2. Certainly sounds like a software issue, which could/should be fixed.