Inscribed Ultimatum - Blood altar trial not functioning correctly

Area: LV65 Ultimatum - Challenge 6

Challenge: Sacrifice Monsters to the Altar by slaying them in the blood circles

No matter how many mobs I kill nothing happens.
Tried all variations of being inside the altar and slaying mobs, standing outside and slaying mobs inside, being inside and slaying mobs outside.

Nothing seems to fill up the altar and there is no visible change when slaying the monsters.

same here 2x. If I get that challenge its a fail