If this is occurring in Haunted Altars of Sorrow, is there a chance that you're being killed by enemy pumpkin bombs (please keep reading)? Not the ones that spawn from killing enemies, but from special enemy variants that just straight up throw them at you? I think those ones specifically register as "Killed by the Architects" when they kill you. Though you said you turned off the Halloween modifiers, I wonder if those enemy variants still spawn when you run that activity in particular.