Sanctuary – “Crimson Tide” [Campaign 4 – Mission 3]

under the control of 2 out of 4 altars.

Something similar happened to me with Inferno, when I had to defeat the mother of the Namtaru.
I don't know where, but I lost one amulet-key (maybe the hero was killed, I don't remember) - and there were only three in the inventory, although the quest said that 4 had been collected.
I was afraid that the gate wouldn't open and I would have to replay - but it worked.
And once there was a bug with the graves of the dead, where you had to find 5 of them. One was visited by a non-main hero, and in the end, all the graves seemed to have been visited, but the quest still said that 4 out of 5 had been found.
I had to replay.