Oh, perhaps I didn't make myself clear. I'm happy that normal service has been resumed \o/
It's possible that the Battery Level peculiarity was present in iOS 14 (and before), but I never looked until things went wonky with iOS 15.
Thus, clearly Apple and their software were the culprit for this, as it tuned out, waste of time.
I maintain that companies should be open about their mistakes. Apple's tenacious refusal to go further than "fixed a battery drain issue" is my reason for the reputation hit; as is the absence of diagnostic tools, which is absurd. There are smart people at Apple, they know this. And they should fix it, else they become just another commodity box-shifter, and that doesn't warrant the Apple Tax.