Trail of Corruption

So I am doing the trail of corruption quest on PoE2, and it keeps bugging out on me. I did it with a friend a few days ago, defeated the boss at the end, but didn't get credit for it. So I reset the instance and tried it again, and again, and again. I've gone through the area several times and usually somewhere between gate 2 and gate 3, the NPC who is supposed to open the gate disappears and doesn't open the 3rd gate, meaning I can't fight the boss, but even if I could, the boss doesn't show up because I've beat him before. What can I do to fix this?