Here’s another quite annoying bug, this time relating to one of the Tormented endboss summons.
The Beast in Ice is summoned by using 
Distilled Fear at the altar. In one of the better updates, Blizzard made the altar reusable so players don’t have to reset the dungeon every time they want to re-summon the boss. It seems this change from Season 5 did have some unintended consequences, however.
A Sneaky Beast
As showcased by Onsokomaru, the beast sometimes just does not spawn after depositing the fear into the altar. It’s not a very common occurrence, but losing any amount of boss mats, especially if you’re running solo, is always a problem.
There may be a way to see it coming, however. Onsokomaru explains that it seems like when the bug is active and you’re about to lose the summoning materials you’re putting in, the frozen people around the altar won’t respawn. This means the dungeon wasn’t properly reset and so that’s why the summoning won’t work. The solution is simple, as you just leave and re-enter the dungeon.
Problesz also added that the bug/non-reset may also be visible if the altar is active, AKA red and smoky with ashes in it. The frozen people seem like an easier thing to spot, but either works.
So save your summoning materials and keep an eye out for the warning signs!