The state of registration of the aircraft determines who may fly it, no one else. Of course the determination must meet the minimum ICAO standards. Any member state may file differences for its own purposes but this has no effect on the agreed minimum international standards.
But bear in mind that if you know that your medical standards no longer meet the required standards of the issuing state of your licence then you must declare this to the issuing state. This is irrespective of the expiry date of the medical.