Probably the best thing to do is start from a save game just before going to the top floor, and try out different approaches. In my case, the quest did end properly. Two were on the roof, two downstairs. Switched to turn based to avoid suicide pathing. First character flew in from the roof. Landed on the top floor, its occupants remained yellow and even had time for a chat. The second one of my party flew in, which was too much apparently, because the fight started. Two from downstairs went up and joined the fight. If I recall correctly, the quest was completed right after the fight ended, and before the police entered and shooed us away.
@ussnorway: In the bugged cases described here it does seem the quest was started correctly.
