I’ve seen occasional heavy drain on both iPad and iPhone. Looking at the Battery preference, it seems the App Store was running (even though I didn’t launch it) and chewing up the battery. I don’t have a lot of apps (compared to other people) on my phone. Pad but not Phone are set for auto updating.
It -feels- as if App Store is doing background processing and possibly updating of the system stuff, but I have no idea if that is the case. (iPhone 12 Pro, 2020 iPad)