When prepayment invoice is posted and receipt is applied to the specific prepayment invoice, status of the sales order is changed to 'Release', (if we have setup 'Sales & Receivables Setup' - 'Prepmt. Auto Update Frequency'). This runs job queue and update the status to 'Release', which is working fine.

When the receipt is unapplied, Sales Order status still shows as 'Release'. It should be changed back to 'Pending Prepayment' because, there are chances where user can apply the same receipt to another Prepayment invoice, which will end-up with 2 'Released' sales orders.