I can just contemplate on my QMB Mission work,
in the past around the time of DBW we Authors (i.e. Bombsaway, ...)
experimented with non standard QMB by altering it's shape into Single Missions like.
Like by adding AI outside the controllable 8 or 16 Flights Sets.
In 4.10 where they where build, they made sense.
In a 4.12 2 or more uncontrollable surplus AI (Fighters) can deplete fun make a Mission
unplayable, despite the Mission easy or goof.
But this does not mean all surplus AI missions can cause this,
a large Number contains easy surplus Spotters or heavy Mavis Seaplanes, these are more Duds
than a real treat.
Basicly yes, it means going into FMB and play with the enemy Flight numbers.
Can be that deleting one makes it playable, or play with the Target set of Enemies,
Better let them set to Ground Attack, in 4.12 any Target set on Flight is hypernatural Matrix style.
Target set to flight worked flawless in 4.10, in 4.12 it's broken.
I never set that crap on.
That's caused by new AI settings. If they are put onto your Flight or even a neighbouring flight, that's forced pancake, and you find that awkward, set it off, don't care about Mission Builders, it's your Game, your Fun, you dictate.
If the Campaign is worth the fiddling, it's not a big deal, go for it.