The battery health does not drain every update, it simply re-checks the physical condition of the battery and it happens to be changing because it's doing it's job.

I noticed the same thing. I tested an iPhone 6s on iOS 14 and compared it to iOS 13. iOS 13 gave me about 4 1/2-5 hours of screen on time with at least an hour of screen off with no issues getting me through a day. After updating to iOS 14, I get 3-3 1/2 hours of screen on time, with 20 mins of screen off.. Definitely a hit in the battery department... I'd give it a few updates, Apple should fix it. Don't forget that it is in fact an early version which comes with issues under that category.