Hmm, what theme are you using? That shouldn’t be happening unless the redness value is above 200, which I (perhaps naively) assumed would only happen for when the answer was incorrect.
Also maybe double check that you’re on version 1.0.2?
https://greasyfork.org/en/scripts/396170-bunpro-disable-backspace-undo
Edit: @joda just published something which might fix it. Try updating to 1.0.3
Thx a lot! I will check it as soon as I’m back from my business trip^^