I was signing up today and when I got to identification step it asked front of my BRP, but when I took a photo it said “ Your ID isn’t the kind of document you told us it would be. Please make sure your ID document matches the type you selected, or go back and select a different type.”. I tried to take a photo of back first and it worked.
Have a look at the top of the screenshot. It says it expects a residence permit. That is to say, if they’d picked ‘passport’ or ‘driving license’ previously, it would say that at the top instead wouldn’t it?
Either this is a bit of an odd thing where the photo is actually on the back, or Monzo have had a hiccup.