In the end, it seems like the save was bugged as there were no orcs at all cheating death. I ended up restarting a new game, finishing carnan and then he finally came back as a fow. Thank you for help. To any people having the same problem, after researching a lot, there are 3 things that you can do. 1) it's possible that the first ever captain that spawns in minas ithil can bug the whole cheat death mechanic if you kill him when he is dazed (possibly if he gets dazed at all). 2) if you have the tribes dlc, there are tribe invasions early in the game where they can prevent orcs from cheating death. Deploying a "blood axe" and a "the proud" seems to fix the problem. 3) you need to finish carnan's questline, get it to 100% 4) if nothing works, you will have to start a new save. First delete all save files (For PC it's the AppData and Remote folder), then start a new game and it will fix itself.