iPad battery drain when powered off (iPadOS 13.5.1, maybe)

There is definitely a problem with the battery life on my iPad. I power it down every night when I go to bed and, somehow, it drains power from the battery overnight.

iPad powered off and it still loses 20% of its battery.
iPad powered off and it still loses 20% of its battery.

You'll note from the battery stats that it's at 100% before it's powered down. Then nothing's using it but it's at about 80% when it is powered back on the next day. That doesn't make sense.

I believe this has only been happening thing I installed iPadOS 13.5.1. It might have been happening since 13.5 and I just didn't notice it, but certainly not longer than that.

The only reason I'm doubting myself about the particular version of 13.5 is because my research uncovered a support thread about it:

But there's also one related to 13.5.1 specifically:

As I understand it, iPadOS 13.5.1 was mainly a security release and shouldn't have affected battery life but, well, I know how these things go sometimes.

I'll update this post if I find a solution.