Add record button edits existing custom field record instead

I just installed 4.7.9 because I wanted the fix for CRM-18988. I am now experiencing a new problem in that trying to add a new record (custom field multiple record) for a contact actually edits the first existing record instead. I confirmed that the bug also exists on dmaster.

Steps to reproduce:

• Create a custom field set 'Custom1' and enable multiple records
• Add a record of type Custom1 to a contact
• Add another record of type Custom1 to the same contact - at this point the form will already be populated with data (from the first record) rather than being a blank form as it should be. Editing and saving it will overwrite the original record.