On the UESP Talk page for A0C00Y07, a couple of people are discussing the issue of monsters spawning outside of the house after leaving. And if you test the quest in classic, you'll find the bad victory check in there too. It really is just a horribly written quest. Fortunately, we have folks like Jay in the community to help fix these quests for us in DFU.
This conversation reminds me of a similar issue with M0B00Y16 "Giant Killing", which likewise lacked any checks to prevent giant waves spawning once player exited dungeon. I had droves of people across reddit, email, and forums reporting this as a bug in DFU - and several of them insisted it never happened in classic. I put aside an hour just to test the quest in classic, and yep - those giants spawn outside the dungeon in classic too (screenshot).
Memory isn't just subjective. Memory is the enemy and each day is a siege.