In the app"Notebooks" by Alfons Schmid, on a new iPad Air 2, while typing, I hid the keyboard. When I recalled it, the letter, and some control keys, were missing. I could backspace, dictate and hide the keyboard, but not type words. Hiding the keyboard did not help. If I quit the program, rather than just switching to another, it seemed to bring back all the keys the first time I used the keyboard, but then the problem was back.
I reset my iPad, but that didn't make a difference. I had no third party keyboards installed. I have never activated my iCloud Drive, but checked and it was off. I disabled the auto dim on my screen, and that was of no assistance.
*** My solution: I checked the accessibility settings - Settings > General > Accessibility on my iPad - and found them all to be off. However, since the Zoom had been mentioned several times, I turned it on. I went back to Notebooks, with Zoom on; the keyboard was still broken. When I turned Zoom off in Settings, that seemed to fix the keyboard. I have not had the problem since. ***
Notes: My problem didn't involve "sticking" keys or delayed typing.
The Zoom I found magnified my screen and didn't seem to affect its brightness.
The reason I posted this experience is that I found just having a setting in the off position may not be enough to avoid the error. We may need to turn the setting on and then off to correct it.