Can't change mob spawns with a datapack

[NiveStinks]

I made a datapack that changes worldgen\structure\spider_dungeon.json to include additional spawns under the spawn_override function. Specifically, I tried make it so that plantain spiders from Neapolitan and bone spiders from Earth Mobs Mod both spawn rarely. I found a spider den and destroyed all the spawners and killed all the spiders, teleported away and back, and only normal and cave spiders had spawned.

After this, I further adjusted spider_dungeon.json to flat out remove normal and cave spiders from spawning at all, repeated my process, and they were still the only spiders that spawned.

Here's the relevant part of the datapack:
betterdungeons.zip