Contact Reference fields not validated when non-empty

Steps to reproduce:

1. Create a Contact Reference type custom field for Individuals.
2. Edit a contact and populate that field with an invalid contact. Save.
3. Notice the validation warning that a valid contact is required. Now select a valid contact. Save
3. Edit the contact again. Enter an invalid string in the autocomplete field. Save.

Results: The contact record is saved without the validation warning. The field 'reverts' to the previous valid contact.

Cause: The hidden autocomplete element ID field is not updated unless a valid contact is selected. This hidden field should be emptied on any change to the text field, so that validation will trigger.