User filtering configuration of custom field is not used in autocomplete on Customer Portal form

If a single user picker is present on a create request form, and the user picker is configured with user filtering, the autocomplete would still suggest all users regardless of the user filtering settings. But when submitting the selected user, the user is validated correctly using the user filtering settings.

Steps to reproduce:

1. Create a single user picker custom field and associate it with the Create Issue screen of the SD project
2. Configure User Filtering to restrict users to only Admin group
3. Include the user picker in the portal form
4. On the Create SD request screen, enter a user who is not an Admin
   • Expected: the user is not shown in the autocomplete
   • Actual: the user appears in the autocomplete.
5. When the invalid user is submitted, the server will respond with "Please provide a valid selection for your ____ Field", which is correct.