You’re chasing the keys on your keyboard when typing on the pad, the most likely cause is this. Face it, it seems weird that as you move a finger towards the key you want, it “whack a mole” vanishes, and comes back intermittently. Go through all the settings, even the more obscure, dwell, motion, accessibility, all have no results. 

a search on the internet might bring you to something like this:

https://osxdaily.com/2021/01/01/fix-iphone-ipad-keyboard-missing-disappearing/

very thorough but often doesn’t solve the problem. If you’ve tried everything with no luck, here’s your fix. It’s the old case you’re using is too worn out. Yep, confirmed on this end, the contacts between the case and pad are making and breaking contact as its held in your hands As you type. That makes the pad think the keyboard in your case is being connected and disconnected as movements against the case are causing the contacts to break the contact with the pins they touch. 

try removing the case. Did it work? If so, you’re welcome. If not, keep on looking. The solution is out there.