I just came across the same bug on Xbox. After Sim Update 6, Leg 12 finally completed at Misselonghi. I proceeded to fly leg 13 but the landing at Kopaida was not registering as completed.
I found a workaround by restarting the previous leg (Leg 12, Aktion to Missolonghi), and flying leg 12 and 13 again in their entirety. I think this cleaned up my save file as the landing at Kopaida registered completed. I’m now able to start Leg 14.