Went and checked on my act 2 save. The hatch is still there and so are the corpses of the enemies in that room.
Check the left side of the painting (not the side of the table). There should be a hidden hatch there. If you can't find it, increase your Wit.
That's the point. I found the second pyramid with a party rerolled to a save before the Dallis attack. Same party, a save well after the Dallis attack, the pyramid isn't in the hidden room.