Why does Apple's Address Book/MobileMe keep renaming my Dad?

I originally had my Dad in my address book with just ‘Dad’ in the first name field. Recently, for some reason, it doesn’t seem happy with him not having a last name and keeps adding it by itself.

It was kind-of amusing at first, but now it’s getting annoying. I’ve tried editing his entry in Address Book and then in the MobileMe control pane, telling it to replace all MobileMe data with the data on my computer. But sooner or later, it ends up with a last name again. I’ve also tried editing it through me.com but it doesn’t keep either.

Any ideas how I can get him to just be ‘Dad’ again?