AFAIK, unless inheritance is working for the PathInfluence column, Fane may not have any special terrain pathing data, which may explain why AI Fane does not avoid Cursed Poison terrain.

The non-Origin undead heroes have these PathInfluence values:

Code
BloodCloudCursed,70
BloodCloudElectrified,100
BloodCloudElectrifiedCursed,100
BloodCursed,70
BloodElectrified,100
BloodElectrifiedCursed,100
BloodFrozen,30
BloodFrozenCursed,70
Fire,70
FireCloud,70
FireCloudCursed,100
FireCursed,100
Lava,700
Oil,30
OilCursed,70
PoisonCloudCursed,100
PoisonCursed,100
SmokeCloudCursed,70
WaterCloudCursed,70
WaterCloudElectrified,100
WaterCloudElectrifiedCursed,100
WaterCursed,70
WaterElectrified,100
WaterElectrifiedCursed,100
WaterFrozen,30
WaterFrozenCursed,70
Deathfog,200

I don't know what the numbers mean exactly, but I'm guessing they're weighted preferences that override hidden values: the higher the number, the more likely the AI will avoid that terrain when choosing a path. Lava is 700, so I guess that means "ye gods, no way am I treading on that!"

I copied the non-Origin undead hero PathInfluence column to Fane in this mod, so hopefully, he won't be too bold. If you have the game on Steam, try the mod. Let me know if Fane's pathfinding improved. If not, maybe the column is properly inherited and these values need tweaking instead.