Problems with the Xardas quest / heart of a Stone Golem

I'm aware that there seem to be some bugs related to the Xardas quest, but unfortunately I couldn't find any solution for my scenario. In my case (PS5), I had already defeated both stone golems (in the citadel's basement and near Xardas' Tower) before starting the Xardas quest. Neither stone golem dropped any loot. I was able to collect the hearts of a fire golem and an ice golem after defeating them.

How do I get the Stone Golem’s heart so I can continue the quest and enter Xardas’ Tower?