I had the same issue and had tried all of the usual suggested solutions. I then found a tip that I had not seen before and it worked for me (for a week now at least). I'll share it in hopes that it works for others as well.
- Turn the power off on the iPhone and leave off for at least 20 minutes.
- Turn the power off on the Watch and leave off for at least 20 minutes. I left both off for 30 minutes.
- Turn the iPhone on.
- Turn the Watch on.
Lo and behold, it worked! The battery went to 65% and ticked down in normal increments without shutting off. I have fully recharged every day since and it the battery continues to work normally. I can't explain why that worked, but I hope it works for others having the same problem.