Only spawns in a very small part of the structure, at least with Fortress... I assume this is the origin point or something.

I'm also getting this problem. Mobs seem to spawn at the cords given by /locate Fortress. Though the only alternative is using:
"dimension": -1,
"block": "minecraft:nether_brick"
though spawn cap rules seem to not function in this mob, and hundreds of the mob spawn within a minute, which is seriously frustrating. And, the only fix to that is by adding " "random": 0.1 ", but they'll still spawn huge amounts given enough time.