The red cloud NPC called "Deadly Spores" (found in the Moldmire) seem to bypass the death magic immunity of myconids. I received a fort save vs death magic, failed, and was killed. If this is some kind of bug with immunity perhaps putting some large saving throw bonus vs death magic on myconid skins/loot might be a reasonable fix.
On a side note, this applies to all -cloud effects- and subraces with immunties to cloud spells (air genasi comes to mind and cloud of bewilderment, etc).
I'm aware of the bug with aoe based spells and immunities not always working, but this is immunity to death magic not applying to an effect which gives you a death magic save.
Because it is a cloud effect, which is why I mentioned it. It might allow a few things to be fixed in one sitting instead of them being skipped over.