I've been playing the official maps as nice quick way of getting a game with friends and family. The mission of choice is 'Escape from Stratis' since it's a pretty simple mission, easy respawn, etc.

I play the map with the AI team mates enabled (as I don't have enough friends - so lonely). I've noticed that the mission will be marked as 'failed' if a user joins as a dead player (i.e. the AI were in control, they died, friend joins, selects that dead player, spawns).

At this point the mission is marked as failed, the current players can continue, but never complete, the newly joined player gets kicked out to the mission failed screen everytime.