Custom vendor bill approval status not changing to approved

I am trying to create a custom vendor bill approval after the bill is created in the system. This part is working fine and the approver gets the approve and reject buttons as expected but when the approver clicks on ‘Approve’ button, the workflow is not changing the status to ‘Approved’ as mentioned in the set field value action in the workflow but it changes status from pending approval back to ‘Open’. Do I need to change any setups to achieve this? I do not think there is anything wrong in the way set field value is used and I am thinking this could be more of a setup issue.

Thanks in advance