The weather fog In all mesa, desert and savanna variants  does not apply when given custom weather fog.

I have provided a custom fog pack that makes all the water surface color fog and weather fog the same so any biomes that have not got the same color can be seen.

{
  "format_version": "1.16.100",
  "minecraft:fog_settings": {
      "description": {
        "identifier": "vocktilesfog:custom_test_fog_desert"
      },
      "distance": {
      "air": {
        "fog_start": 0,
        "fog_end": 0.85,
        "fog_color": "#7354ff",
        "render_distance_type": "render"
      },
      "water": {
        "fog_start": 0,
        "fog_end": 20,
        "fog_color": "#ff54ee",
        "render_distance_type": "fixed"
      },
      "weather": {
        "fog_start": 0,
        "fog_end": 100,
        "fog_color": "#54ff7c",
        "render_distance_type": "fixed"
      }
    }
  }
}