Could it be that your Actions (Change Status to Assigned) is triggering the Approval Process to no longer be blank?
Have you tried removing the Status (Approved) from your conditions just to see if you can get a blank approval process to trigger the action?
Have you moved this particular Action Rule to #1 in the priority, just to eliminate any potential that another Action Rule is superseding it?
I'm not sure an Action Rule is going to get you what you are looking for anyhow. You may have better luck controlling this through Tech Permissions.