Noticed a group doing it today who got the journal boss-kill update while the boss was alive. Looked like they got it JUST as they reached the large area.
Confirming that this happened to me 2 days ago. I thought my summon had somehow ran ahead and killed the boss.
Yup, happened yesterday to me.. Before we got to the large area as well.
I've seen it happen twice at least. Not sure if it's tied in to location - I think it might be happening when you kill one of the regular ghouls near the boss area. At least, I think that's when it triggered for me at least once.
This does indeed happen, and it happens as the first in the party crosses in the doorway between the last tomb section and the ghoul feasting area.
The on_death script that indicates you completed the quest was in the on_spawn event instead. Oops. Fixed.