Had this happen to me too. Specifically, in the rat's nest mission in zhufbar. Editing the save file and army.mrc to try to fix it doesn't work, and cpl. Fletcher's stats are all set to 0 in the troop roster. From this point on in the campaign he begins every mission by dying during deployment. This seems to be related to the armor rating of the unit being upgraded from 2 to 3, while the unit is at least at 2000 experience or more.
I don't want to redo the previous mission, but I also don't have an easy fix at the moment. It might be as simple as "don't bring level 2+ fletcher to the armor upgrade mission in zhufbar." Anyone got some suggestions?