Softlocked on Cruel Difficulty Legacy of the Vaal Quest

Was doing the Legacy of the Vaal quest to get the large soul core and bring it back to the Stone Altar, but got sidetracked by an inscribed ultimatum drop.

After going to the Trial of Chaos and completing the ultimatum, the game unknowingly sent me back to act 3 and upon going to the stone altar (in act 3) I saw that the altar already had the soul core in it and got confused, while trying to figure it out I dropped the large soul core on the ground where it instantly disappeared.

Now going back to cruel difficulty, I can't use the stone altar since I don't have the soul core, and the boss doesn't drop another one, so I can't progress in the game.

I have the exact same problem, did you manage to figure out how to proceed?

Edit: Solved it by joining a party and letting them progress the quest.

i have the exact same problem

Exact same problem. Cant solve it tho since im playing ssf