So I was at the 3rd purifier and after eating the boss at District 6 I got disconnected then fought it again for the 2nd time then I died, after that I went again to the third purifier but then, I can’t access it anymore nor can’t fight any boss I can just see a blue flag on my navigator, tried using gamepad mouse and keybord nothing worked please help 
Not that I’m knowledgeable on this bug, but have you tried abandoning the quest and starting it over by talking with the 3rd purifier? Just trying to help 