When i entered the volcano dungeon with a mercenary everything was quite fine but when i left the volcano dungeon i was hurt so i decided to wait. And i think my mercenary was inside the volcano cause after i waited i just get a message that my mercenary´s died mostly likely because the volcano is blocked so he could not get out.
The whole ai spawning somewhat randomly might be hard to fix at this point but there could be an easier fix to this like flowing lava from the volcano so your mercenary could get out if it spawns within the volcano.
But the fact is im not sure what killed them but i could not see any stuff or corpses and i cant see inside the volcano so all i can do is guess that there is lava within the volcano.
This just a rather small thing so no hurry and i hope that people learn that mercenarys might die at the volcano so you should not wait in the volcano in case you´r mercenary is inside the volcano. (Update i found a Pickaxe and dug into the volcano and found the crpses so they both died inside the volcano that is outside the dungeon.)