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.

Update - 25 July 2020

This has been cured in iPadOS 13.6. My battery no longer drains when the iPad is switched off. Other things have changed with the power management too. For example, merely plugging in the iPad will boot it up. Previously I had to plug it in and press the main button to power it up.