Thank you for the feedback and the description. The error message you are seeing is what it implies, i.e. it is checking that the digit is P,M or D and that it is followed by a number (any number). This code has been under heavy development and testing in the last few weeks. I am sufficiently confident in it that it has been going through the plugin approval process so it is a bit of a mystery.
The closest I can get to what you are seeing is to put a space in front of the criteria id (i.e. the P1, or M2 or whatever). And I am going to tweak the code so there is a trimming of any leading spaces in a code update. It will not be a browser issue and it will not be a Moodle version issue, but it could easily be a code version issue, i.e. you might have a version from a few weeks ago that has a bug in it. If I had a copy of your your file from the folder (where moodle is the root of your install)
mooodle/grade/grading/btec/lang/en/gradingform_btec.php
I would have a good idea what version of code you were running