I have found the same as GP. I have an iPad Mini Retina (2014?) and an iPad Pro (2018). I once ran a test with both of them in airplane mode, and the Mini outlasted the Pro by a mile.

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.