I'm still having weird issues on iPadOS 13.3 with battery life on my 11" iPad Pro. This has been a problem for the last two or three releases. 13.3 made it a tad better, but I'm still frustrated.
Last night I hard rebooted it and put it on my desk at 35% battery before going to bed. After 7 hours in standby, it went from 35% to 30%. Not too bad--I can live with that. Still worse than it used to be pre-iOS 13, but not terrible.
But then from 6 am to 8 am, it went down to 25%. So 5% battery drain in 7 hours of standby, and then another 5% drain in 2 more hours of standby. I just looked again, and from 8 am to 11 am it's drained another 3%. In other words, 10% of standby drain every 12 hours, 20% on average every 24 hours. My iPad would only stay on for 4 days total on standby before the battery died.
A couple of weeks ago, I was so frustrated with this behavior that I did a DFU restore without copying from backup, so this is fresh install of the OS. Even after that, I can still never tell when the standby drain is going to go crazy.
This issue corresponded with an iPadOS update (I forget which exact version of 13.2 it was that this started happening) so I know it's not hardware related. And in the battery menu, it never shows anything running in the background, other than about 1 minute per hour for Siri, and that doesn't even show up in every hour.
I guess it's just annoying because back in the day, iPads would hardly drain at all while not in use. That's definitely not been the case on the Pro models for me, no matter what I try, and it's been especially bad on iPadOS 13. Incidentally, I'm not having this issue at all on my iPhone 11 Pro Max.