iPhone keyboard not working for just certain letters.

I have an iPhone 5 SE, running iOS 12.1.4. Recently, when typing I've noticed several letters on the keyboard simply don't work when the phone is in the upright position, specifically the letters "n", "b", and "u". If I rotate the phone to a horizontal position, those same letters will (occasionally) start working again.

I have tried rebooting the phone, but the problem still continues. I have a Zagg Shield glass screen protector on the phone, but I don't believe that is the cause of the problem (it's been on for years without this problem appearing).

Has anyone else experienced this same issue? And how did you fix it?

Appreciate the help. This is maddening!