Yep, I also guess it's the device encryption then. Not necessarily Knox (at least I could disable each Knox package without any problems).
What's your exact firmware code?
There are a couple of apk's who can drain battery if you deleted them.
I don't remember their name but if you debolat them the phone won't get into deep sleep....
But you can always double check if the phone enters deep sleep by installing CPU Spy from Play Store.
This might be what you are looking for. I would imagine the same process will work on an s21.