dudeee i was about to ask what to do lolI want to say that I also experienced this bug.
After finishing Nadia's current storyline, I was teleported to the crimson apartments, experienced the alternate sex scene (the one in her apartments) and then after the scene, your party is missing Nadia and you are alone in the East adventure map.
Now I get a game ending dark screen after I do her sex scene in her room (At the Grid town) at night
and yeah after i try enter nadia room at night i got the scene and isaac bangin his teacher and quest finished, finally!
i got so confused when we wake up alone in nadia old apartmen room, all the npc gone and the quest is't finished yet lol