The portion of this achievement, which is also a portion of the new Meta Achievement, is impossible to complete. The Forbidden Hoard spawns below its platform and it is literally out of reach. You can see it if you angle your camera right, but the actual chest itself is impossible to grab. I’ll move this to bug report too, but just spreading the word if it hasn’t already spread.
The forbidden horde spawn in a bunch of different places… if you are talking about the one in the lava room, yes i have seen it spawn below the floor along with the loot goblin as well.