Found no Siptah bosses in Wine Cellar or the volcano dungeon.

BUT one of the serpentmen bosses dropped a Siptah weapon, the Blade of the Betrayer. So, maybe there aren’t new bosses in all dungeons, but there is new loot at least in some of them.

Also, the increased drops from the mobs and chests make dungeon farming pretty efficient, I think. Got a boatload of improved healing potions from Wine Cellar, for example, 60 or so in total.