I'm playing the Definitive Edition and I can state that it's not bugged. On the Nameless Isle I cleared 5 of the 7 altars the normal way (by doing the quests associated with the respective altars) but I obtained the solutions for the final 2 altars by doing the quest for Alexander. But when I went to explore and pray at these two final altars the achievement unlocked for me after I finished the seventh.