I haven't amended the field, it is mapped to the contact record on the core profile field and on the order form. The data is getting stored, it is visible in the client record in the back end, it just doesn't show on the front end.
The problem does not occur on your demo site.
Thanks for your help.
Update: This is very strange. It only happens when I use "Sid" as the first name. Anything else that I've tried so far, it doesn't happen. It doesn't happen on the demo site either.