Phone Numbers Deleted When Sending Text Using Google Sync'd Contacts

my iphone 5 on iOS6 has its contacts driven by a sync from Google. It uses the CalDAV method as different from iOS5 and the sync setting is a different one from the Mail and Calendar, as it seems everyone says to use. Recently when i send a text to a contact, the number i use in Contacts often disappears. On closer inspection the contact i use is still in my contacts but just the phone number in the field of the contact has been deleted automatically. Its also disappeared from Google Contacts when i check on Google.

Odd. but any ideas?