The key for the audit trail is the RM #. See the record structure in 1.4.13 of the Pax IS-IDEC document, where, for the 2nd and 3rd rejections, you need to provide "Your Rejection Number" (element 17).

Thus, in a situation in which the OA bills with an incorrect ticket number, you'd reject, referencing that ticket number. Their rejection would reference your RM #, and the audit trail would stay intact.