Campaign persistent variables are set at debrief init, not (as we previously believed) at the end of a successful debrief. This means that if players failed the mission (without dying) and went to a failure debrief screen, all their air wing losses would be added to the CPV.
We actually had a fix in place in the mission file for more than a year, but we never figured out how to repro the bug so we weren't able to check whether the fix worked.
e: basically the problem was this: everyone believed CPVs were only stored at the end of a successful campaign mission. Turns out that's not true.
e2: Come to think of it, that means a lot of our other campaign persistent variables are probably ****ed too.