Same issue here. Had an iPad Pro 11 2018, was losing 10% battery on standby per night, had it checked in store and battery was in good health, no fault found. Upgraded to M1 11" Pro in store as I'm sick of trying to solve the problem and have the same issue again. Store insisted it was a software issue caused by corrupted backup. So set up new device as new, no backup restored.
Have narrowed it down though. Suspects were Magic keyboard, and Safari... eliminated Magic keyboard as cause by leaving it on standby with keyboard connected, no apps open. Little to no drain. Opened ONLY Safari and it's lost 5% in 5 hours, zero minutes screen time.
At this stage, I'm thinking I'm going to return it and ditch the iPad completely. I don't use it every day, so consistently come back to it every few days with half empty battery and burning cycles doing nothing. I don't see any of these issues with the MacBook Pro. I don't really want to have to remember to close all my apps or shut down and restart my wifi every time I use it... if I wanted a complicated life I'd just go with Samsung or windows. So annoying. Had iPads for years and they just used to last for weeks on standby.
Only issue is I traded the 2018 model for store credit, so have no idea how the refund situation would work now.