I actually built two Shortcuts to simultaneously toggle low power mode and all wireless connectivity. I have an older iPad I use for videos and podcasts while exercising that lasts over a week by using those, with zero power usage in between sessions.
My hypothesis is that the Mini still runs iOS, and the Pro runs iPadOS. I think I started noticing the Pro standby battery drain after "upgrading" the Pro to iPadOS, whenever that came out.
The way it works is that the Bluetooth and GPS stacks are powered on every few seconds briefly, even if the device is supposedly off.
iPhones that have "home" buttons demonstrates this not-true-sleep very well. The button looks like it is embedded in the front glass very well with a nice satisfying click when it's depressed...
...except that it's not a button at all. It's just a polished circular indent in the glass! There is no moving part.
The click? That's haptic feedback from the vibration motor that works even if the phone is "off". If the phone loses all power or is truly turned off, then suddenly the button goes dead and you realise that it's just smooth glass. I had that phone for years before I first noticed this.
I find that having an Apple Pencil 2 snapped on the side makes the iPad drain its battery.
It wasn't always like this. We still use an iPad 2 and 4 for CC processing and they don't need to be charged for 2-3 months at a time!!! It is definitely on newer devices. I have a Yoga 1 that i can have in sleep for almost a month and I can still have it turn on with some percentage of battery i can do a few things.
What Microsoft and everyone else it seems, is doing is maddening, the batteries are getting worst, and you simply cannot just rely on a device to be usable anymore without being next to a power outlet and probably even waiting 15minutes before they can even power up.