11-26-2012, 08:04 PM
Okay, I've solved the problem after two days of trying and so many syncs I've lost count. I just don't know why what I did I solved the problem.
Here's what I did:
Created a new entry in Address Book in the same group.
Added the surname and initial (to be easily distinguished from full name in the original record), phone numbers, email address and birthday from the original record but ignored the other data.
At this point what I expected to see was a duplication in Contacts on the iPhone which mirrored the two records in Address Book. The record with the initial did not appear. I assume the Contacts app is doing some automatic de-duping by comparing a number of fields. The original record was still there, with all the information and the correct birthday!
Wondering if it was doing some sort of on the fly merging I deleted the incomplete record in Address Book and synced again. No change. The birthday is now correct in Contacts and editable on the iPhone.
My original idea had been to replace the contact with a newly created one after checking there wasn't some sort of problem with the specific date of birth but it seems this was enough to fix the original problem.