If you have any of the screen settings set, like dark mode settings, Night Shift mode, Tru-tone settings activated, these can affect the brightness of the screen all the time.
I am not sure if this is a bug or not.
Since iOS 14, I, also, find myself constanrly adjusting the screen brightness all the time as the screen brighnress constantly jumps around during use.
It may, also, have something to do with the ambient light sensor on the iPad, as well.
Again, not sure what is the culprit for this.
If you toggle off the auto-brightness setting, you lose the dark mode features and time set-ups to activate and de-activate dark mode.
So, I have to leave the auto-brightness setting activated.
It has been annoying, but I have still not found a fix for this screen brightness issue.
My wife has been dealing with this same screen brightness issue, as well, on her 7th gen iPad, since I upgraded her iPad to iPadOS 14, as well.
Frustrating as you need to keep a constant vigilance on the screen brightness setting all the time you are using tbe iPad.
Have not found a solution to this issue.