UserPicker shows "Add new customer" functionality when used as custom field

The UserPicker is showing the "Add new customer" functionality when used as a custom field (see screenshot). This should not be possible for custom fields as unregistered email addresses can only be used through "Raise on behalf-of" or "Share request" fields. Full details here: https://bulldog.internal.atlassian.com/browse/JSDSJAC-226

How to reproduce:

1. Create a "User Picker (Single User)" custom field and associate it with the Service Desk screen
2. Modify a Request Type in the Service Desk so that the form incorporates that custom field
3. Configure the permissions as per screenshot (allow anyone to raise request and search in the portal)
4. login to the Customer Portal as admin and raise a new request, when filling up the "User Picker (Single User)" field, type a random email address that is not an existing customer.

Expected:

Nothing should happen and the field should be marked as invalid when submitting the request.

Actual:

The dropdown shows an option "Add new customer <email-address>" allowing the customer to select it let him think that it is possible to do so.