Nor will anyone hear of this, as I suspect that the few of us who have had issues are coming up with something that Apple employees are not being told about as a collective issue.
I had the same issue and it took months for me to fix the issue after being suggested a fix by someone who used to be a Senior Advisor. After about three weeks since the reset, I decided to shut down the watch after fifteen minutes after noticing the charging speed was not consistent. It went from 100-94%, and eventually charged to 100%.
Honestly, this might just be an issue with people never shutting their watches off, causing some sort of internal issue that builds up over time. It may have occurred in the past but not been as dramatic, but later software updates to make things more accurate only made things appear worse. That our watches have been so dramatically off is just an extension of this, built up over time. The issue is almost certainly in our phones, as those of us who have received new watches still have the new issue appearing.
My best advice is to periodically shut down the watch at least once every two weeks to reset things, and go from there. The last time I reset after a reset, I went about three weeks before I fixed things, so a reset two and a half weeks has now given me any issues as I close in on three weeks.