I have a structure, and sometimes several mobs spawn there. This is a boss and must spawn alone.

Here is the code for spawning entities in the structure

{
  "type": "minecraft:jigsaw",
  "start_pool": "egyptcraft:boss_pyramid/start_pool",
  "size": 5,
  "max_distance_from_center": 80,
  "biomes": "#egyptcraft:has_structure/boss_pyramid",
  "step": "surface_structures",
  "start_height": {
    "absolute": 0
  },
  "project_start_to_heightmap": "WORLD_SURFACE_WG",
  "use_expansion_hack": false,
  "spawn_overrides": {
    "monster": {
      "bounding_box": "full",
      "spawns": [
        {
          "type": "egyptcraft:anubis",
          "maxCount": 1,
          "minCount": 1,
          "weight": 100
        }
      ]
    }
  }
}

Maybe it's something else?

One quick solution I can think of is to slightly light up the structure so regular mobs can't spawn (they only spawn in complete darkness).
Another useful way would be to look at some vanilla structures that doesn't spawn mobs other than the one related to that structure, like the new trial chambers in 1.21 (you can look if there's something particular in the structure json file inside a snapshot jar file) or maybe the end cities (where only Shulkers spawns inside, not even Enderman if I remember correctly)