Set status property is jira.permission.assign.denied but still able to change assignee to Automatic

My intent is when an issue in Closed state then all the fields cannot edit
I have searched and found solution is set status property of CLosed is

then most of the fields cannot edit as expected except Assignee. I can select and change the value to Automatic
then Assignee is now Unassigned and cannot change

Is this a bug or incorrect configuration?
Is there any configuration to set un-editable in Closed state
Thank you!