Hi,
But What I really don t understand is why KDM only appeared in this case in MIRO
As your client is using SAP since a long time and never happened this- as this KDM key appearing now on MIRO posting, check any configuration done, any TR moved to P system which cause your concern or earlier all your MIRO posting done in same month and now only your posting MIRO done in current month for previous month transactions related PO/GR.
Regards,
Biju K