Contact subtype overwritten when profile submitted containing custom fields from other subtypes

If a contact is marked as Subtype 1, and they submit a profile form containing custom fields specific to Subtype 2, the contact is changed to a Subtype 2, rather than this being added as a second subtype.

Replicated on WordPress demo:

1. Create a contact with a random email address, and assign them the 'Staff' subtype
2. Create a custom field for the 'Parent' subtype
3. Create a profile that includes the email address and the 'Parent' custom field. Set this profile to 'update the matching contact'
4. Test-drive the profile, entering the email address assigned in #1
5. The contact will become a 'Parent', rather than both a 'Staff' and a 'Parent'