I created a user picker cf to designate the QA resource for a ticket. The field is NOT required in my field configurations. But when I attempted to clear this field after a user had been selected, the field did not allow a null value. It returned the deleted value to the field. In a related issue, when I tried to to a bulk change to migrate multiple tickets from one issue type to another, the wizard wanted to require me to set a user for this field as though it is required. I cannot find any reason for this behavior in any of my configurations.

