Hit (i), hit the pencil by “Mnemonic”, but don’t hit “save” or “X”. Just click away from the info box to cancel it. On future reviews the Android OS will pop the keyboard (probably as a result of modifying that text box and having it be editable again, even though it’s invisible).
I’ve hit this a few times but this is the first time it was obvious to me what I had done.
(And the reason I click pencil from time to time without editing is that I keep thinking it’s going to give me options for other mnemonics when the one that’s there is terrible)