Not sure if this applies to other types of forge map in monolith, but on more than one occassion, ground AoE poison from the Acid Forge mobs do not show if close around the forge itself. Changing position of the character (running around and further away) shows the poison AoE affects flicker visble and invisible until they expire. The AoE is still active even when invisible to the player.
In some places it looks like the effect is below the surface of the visible map. i.e. the ground seems to be higher than the Aoe poison green. This is more pronounced when the Acid Forge spawns close to an area which has a change in elevation.