With most batteries that’s entirely normal, or so I’ve learned from LTT. My background is linguistics and law enforcement, so it’s 2nd hand knowledge and from YouTube, but seems legit
Not observed it directly on my S5 watch as I’m a light sleeper. I take it off and put it on the charger when I go to bed, switched off, and just reach out and flick the switch around 4am when, as I do 99% of nights, I pop awake for absolutely no reason. Presto, fully charged watch at 6am when the alarm I’ve been waiting for for 15 minutes already goes off