Area [Underdark - Foul Hollows - Exploration] seems little buggy with red mushroom spores.
Our group was triggered red mushrooms when we was on other part of map without red mushrooms.
Here is screenshot which I hope it can explain where we was and from where red mushrooms was triggered.
(https://pp.userapi.com/c847019/v847019682/cf4d3/gzqi_XEIn8o.jpg)
I am not sure how to recreate this bug.
Two characters was standing and walking around marked place on screenshot. And from other part of map red mushroom shot cloudkill.
As one of the people killed by this, I can add that we were stepping around near the transition and saw "Trap Triggered" and the spell shot from across the map (from the mushroom to the NE)
Claudia died to this once.