I've seen some other threads on this and haven't been able to figure out how to make this work myself. Here's the breakdown:

I'm adding a mob that doesn't have configurable spawns to the Spawner.json in InControl like so:

{
"mob": ["enemyexpansion:caster"],
"persecond": 1,
"attempts": 10,
"weights": 80,
"amount": {
"minimum": 1,
"maximum": 2,
"groupdistance": 3
},
"conditions": {
"dimension": "bro:saturated",
"inwater": false,
"inair": false,
"mindist": 40,
"maxdist": 128,
"minheight": 60,
"maxheight": 160,
"maxhostile": 60
}
},

This part works fine, the mob spawns in the custom dimension, they spawn in every custom biome. I need to force them to spawn in ONLY the biomes I want to specify and I'm trying to do that using the Spawn.json file like this:

{
"mob": "enemyexpansion:caster",
"biome": "bro:saturated/the_enchanted",
"result": "default"
},
{
"mob": "enemyexpansion:caster",
"biome": "bro:saturated/stranger_flats",
"result": "deny"
}

One rule allows them in a certain biome, the other denies them in a different biome. The problem is that this doesn't work at all, the mob still spawns in every biome in the custom dimension. I've tried a bunch of different configurations, I've checked the biome names, nothing seems to work. Any help would be appreciated. Thank you!

Remove "biome": "bro:saturated/stranger_flats",
in the second rule. You want to deny in all biomes except the first

Thanks for the response! I had tried that before and was still seeing mobs spawning outside of the specified biome. I just tried it again and they're still spawning in every biome in this custom dimension. The Spawn.json file now reads:

{
"mob": "enemyexpansion:caster",
"biome": "bro:saturated/the_enchanted",
"result": "default"
},
{
"mob": "enemyexpansion:caster",
"result": "deny"
}

Haven't changed the Spawner.json file.

I've tried a bunch of different configurations, removing the dimension condition from Spawner.json results in no spawns in the dimension at all. Using "onjoin": true in the deny result rule stops all spawns everywhere no matter what. I'm not sure if there's a way to solve this but wanted to bring it to your attention anyway.

Sorry for slow response:

Did you try adding "onjoin": true?