Honestly, I think it's a combination of things. I will do the best I can to sum this up in a way that makes the most sense, because I have had quite a bit of experience with this issue:
I had battery issues with my 2018 11" iPad Pro. I used it exclusively with the Smart Keyboard Folio, and never had the Magic Keyboard for it. Starting with roughly iPadOS 13.3.1, I started noticing pretty severe battery drain in standby. It was a pretty abrupt change in battery performance after one of those 13.3 dot releases, so I noticed it right away. I tried everything I knew how to do to fix it, including a simple reset and restore from backup, a reset and manual reinstallation of all my apps without restoring from anything, a DFU restore and then restore from backup, a DFU restore and then a manual reinstallation of all apps, and then finally a call to AppleCare.
AppleCare ended up sending me a replacement iPad Pro, saying that their technicians found that the battery was just worn out. I had a battery actually go bad in my 10.5" Pro and the replacement did not display the same problem, so I believed them that this was mostly likely the issue. In doing a quick lookup, I was able to determine that the replacement item I received was manufactured about 2 months before I received it, so I was pretty confident that it was brand new and not a reconditioned unit. Since Apple claimed that it was a battery issue and since my backup was pretty fresh, I decided to restore it from my most recent iCloud backup. Long story short--the old standby drain issue came back instantly. Even after giving it some time to settle in, it was draining roughly 1% an hour in standby. So if I took it off the charger with 100% charge at 8 am on a Monday, by 8 am on Tuesday, it could be down in the 80s or even 70s. I tried another fresh reinstall of everything as well without backups, and got the same result.
I tried to do things like keep the Pencil detached when not in use, take it off the keyboard folio for the night, and a myriad of other things. The only thing that ever worked reliably was to turn Wifi off when I wasn't using the iPad. When I did that, it would drain only 1 or 2% a night in standby, which is what I feel should be the normal amount of drain in 8 or so hours.
I will note that throughout this entire process, no app ever showed any excessive battery usage in the background in standby. Siri would show up in the chart over night, but not every hour and not for enough time that it would cause any significant drain. I confirmed this to be the case by turning Siri completely off and saw no change in performance.
Eventually I upgraded to the 2020 12.9" iPad Pro and sold my 2018 11" iPad Pro. I never figured out the issue, but after all my extensive troubleshooting, I feel it was being caused by one of the following things, or more likely, a combination of them. Again, these are just my theories and aren't based on anything Apple has told me:
--My battery was coming up on 2 years old, and could have been contributing to the problem.
--An issue with iCloud sync could be causing the Wifi radio to fire off too often during the night and not doesn't let the iPad fully go to sleep, at least not for a while after you first put it to sleep. If you keep opening the cover and checking the battery level, you keep waking it up fully and starting the process over again. This is probably why turning Wifi off essentially fixes the drain issue.
--The iPadOS updates that started causing this were the ones that addressed apps closing in the background when they shouldn't. I have to wonder if the fix for that issue is partially causing the drain because it's having to use power to keep those items open in RAM.
--The simplest explanation: iPadOS is now just generally more battery hungry than it used to be. You'll notice that on the iPad Pro specs page, it no longer lists standby time on the battery--just usage time.
As for the 2020 iPad Pro with Magic Keyboard that I use now, it's still draining in standby way more than iPads used to drain in the old days. I did a from-scratch install of all my apps when I got it, so I didn't pull anything over from my 2018 11" model, and I'm losing about 10% a day on standby. This is without any apps showing that they're causing any significant drain in the background while sleeping. It could be that there's a rogue app doing this, but if there is, I can't figure out what it is. I've just chalked it up to being an improvement over what I was seeing on my old one. The latest update from last week also seemed to improve things a little more. I'm hopeful that Apple is getting this fully under control little by little.
