No, if you request the deletion of a document in ApprovalMax that has previously been matched, the removal of such a document will not lead to auto-unmatching.
In order to unmatch documents once matching is closed (i.e., the Bill has been approved, the Purchase Order is marked as Billed), you should void the respective document in Xero - it will then get marked as "Cancelled" in ApprovalMax. Now you can request the deletion of such documents in ApprovalMax.