I believe there was a change to the event whereby 2 players could trigger the boss fight. Originally, it took all 4 to be present in order to trigger the fight. The change was absolutely necessary as 1 player searching for Voca could delay the whole squad. But, the change also opened the door for people to afk the mode.
I haven't noticed any increase in people departing mid-mission, but the afk folks have definitely increased.
I'll take afk players over 1 explorer delaying completion of the mission.