Status is an indicator of where an issue is in its lifecycle, it has nothing to do with the assignee.
"Status = assigned" is the wrong approach here, just because you assign it or unassign it, doesn't mean the status should change. The assignee is who owns it at the moment, not where it is in the workflow.
If you insist on doing this, then you will need to find/write a listener that can spot "assignee changed to empty" and trigger a transition from <any status> back into Open. You'll need to change your workflow to include the required transition(s) as well.
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