It's a pain that we are having this on premium phones.
I never had this kind of unidentifiable issue on Android (on my old Xperia XZ2, Samsung Note 4).
I even though I always do a clean restore (with no backups) every major update (like 14.8.1 -> 15 or 15.4.1) just to make sure that everything is fine, sh*t still happens.
Sudden battery drains, overnight drains, back heating up, etc.
I turned off most possible background things like the Motion & Fitness thing, reduce motion, turned off the background app toggle thing, turned off all related to Siri (all per app toggles and Siri toggles), uninstalled all apps except the only ones I use (even the Apple Calendar, Notes, Mail etc), Notifications are also turned off for those unneeded sh*t except for chat apps, I'm even on Zoomed display, all auto-updates are off, charhe my phone around 40 to 90 only, and every nook and cranny of iOS settings and per app settings are optimized.
It comes to me to stereotype that iOS operating system devs are incompetent bastards that only know how to do UI and such.
And that "wait for a week for the indexing to cooldown", like what the heck? iOS has the "most" powerful CPU, it shouldn't be like that unoptimized!
Anyway, I'm holding off until the last update for iOS 15. My wife and I are still on iOS 14.8.1.
Well, I'm actually contemplating to update to 15.4.1 before my wife does but reading this thread makes me want to postpone it again. She'll kill me if the same iOS 14.0 fiasco happen on her phone.
I don't much care about security updates for now as I am tight secured with my household system and as a poweruser, I know what to not click/do.
edit: spellings