What happened just a few minutes ago from this posting:
Siauliai Mission. Great party, smooth run and all, then last boss (the tree). Its HP was way higher than usual, it took a lot to bring it down. And then, instead of dying… IT REFILLED THE WHOLE HP BAR.
Everybody got confused and laughed but we decided to screw-and-kill it again anyway. But then, a few seconds later, everybody was expelled from the mission. No exp, no money, no useless red box.
As I thought I was the only one expelled, I decided to run back to the mission. The boss was still there. But alone. No one else came back.
/sadstory
I suspect it may have glitched because one of the party members used many exp cards and leveled a lot at the beginning of the mission.