My pipeline intermediate result looks like this:

[
  {
    "claim": [
      {
        "_id": "1-8101-2023-0445722",
        "data_accadimento": {"$date": "2023-11-30T00:00:00.000Z"},
        "index": 180,
        "rainValue": 26.290000000000006,
        "hailValue": 0,
        "inundationValue": -1.7223792589410434,
        "windValue": 5.940000057220459,
        "lightningValue": 0
      }
    ]
  },
  {
    "claim": [
      {
        "_id": "1-8101-2023-0445722",
...
]

I am trying to access rainValue inside a $project stage by doing:

{
    $project: {
        claimRainValue: "$claim.0.rainValue"
    }
  },

However the result is an empty array.

Why? The number of elements inside the claim array is one.