Just tried again. Interestingly the address they had for me on file was a rather truncated version (e.g. first line, London, Postcode) and they had also truncated the first line of the address (had put 123 Thenameofmy House as 123 Thenameofmy Hou). When I didn’t amend their record in the slightest and just clicked okay it went through without issue.
Have just successfully added the debit card to TFL. Used my First name and Surname in the field “Name on card”. My address with Monzo matches that with TFL. As @alexs says this doesn’t seem to be a PAN issue.