I've noticed that in some missions, like Infected: Manus, players are literally leaving the mission a couple of seconds after they enter it. Even when the mission hasn't started yet. At one time this resulted in us having to complete the mission with only 3 people.
I don't know the reasons for this behaviour, but I think it's safe to assume that leaver penalties are not working here, as I've noticed that specific persons have been doing this multiple times for weeks now. They join, and immediately leave, not once, but multiple times.
Also, once the room with Ogen is reached and as soon as he's killed, players tend to leave cause there's no penalty here anymore. In other missions it depends on how many people have left before you, and even that's not very reliable: sometimes you get a penalty while someone else has already left the match, other times you don't.
Consistency is needed here. Leaver penalties are not working in some missions, and overall the mechanic seems very unreliable.