Lower case 'r' followed immediately by a number (string of digits) dissappears

If I add a comment to an issue, for example, and type a lower case 'r' immediately followed by a digit or string of digits, the sequence is not visible in the comment when it's not in 'editing mode'. If I re-edit the comment I can see the test as I typed it. Using a capital R is fine - so R12 works, but small r12 doesn't. This seems to be related to comments only as this description seems fine.

I cannot find an reference to r followed by digits in the formatting help so this must be a bug.