Why doesn't the compound wildcard index work as expected?

Working with Data
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1

I created a test collection with the following documents:

db.products.insertMany([
  {
    "name": "Smartphone A",
    "category": "electronics",
    "price": 4999,
    "attributes": {
      "brand": "Brand A",
      "color": "black",
      "storage": "128GB",
      "screen": { "size": 6.1, "type": "OLED" },
      "camera": { "main": "48MP", "front": "12MP" },
      "specifications": ["5G", "waterproof", "fast charging"]
    }
  },
  {
    "name": "Smartphone B",
    "category": "electronics",
    "price": 6999,
    "attributes": {
      "brand": "Brand B",
      "color": "blue",
      "storage": "256GB",
      "screen": { "size": 6.7, "type": "AMOLED" },
      "camera": { "main": "64MP", "front": "16MP", "features": ["night mode", "ultra-wide"] }
    }
  },
  {
    "name": "Laptop X",
    "category": "electronics",
    "price": 8999,
    "attributes": {
      "brand": "Brand C",
      "color": "silver",
      "processor": "i7",
      "memory": "16GB",
      "storage": { "type": "SSD", "capacity": "512GB" },
      "graphics": "dedicated GPU"
    }
  },
  {
    "name": "Sport Shoes Alpha",
    "category": "clothing",
    "price": 899,
    "attributes": {
      "brand": "Brand D",
      "color": "white",
      "size": 42,
      "material": "mesh",
      "features": ["shock absorption", "lightweight"]
    }
  },
  {
    "name": "Coffee Machine Pro",
    "category": "home appliance",
    "price": 2499,
    "attributes": {
      "brand": "Brand E",
      "color": "black",
      "type": "capsule",
      "functions": ["preheating", "auto-cleaning", "programmable"],
      "capacity": { "water": "1.5L", "beans": "200g" }
    }
  }
]);

Then I created a compound wildcard index:

db.products.createIndex({ "category": 1, "attributes.$**": 1 });

When I run this query:

db.products.find({ 
  "category": "electronics",
  "attributes.color": "black",
  "attributes.brand": "Brand A"
})

Why doesn’t it use the wildcard part of the index to filter on the attributes?

The execution plan shows the indexBounds as:

{
    "category" : [
        "[\"electronics\", \"electronics\"]"
    ],
    "$_path" : [
        "[MinKey, MaxKey]"
    ]
}

Instead of what I expected:

{
    "category" : [
        "[\"electronics\", \"electronics\"]"
    ],
    "$_path" : [
        "[\"attributes.brand\", \"attributes.brand\"]"
    ],
    "attributes.brand" : [
        "[\"Brand A\", \"Brand A\"]"
    ]
}

Please let me know If I’m not clear enough with my question. Thank you