sortKeys() doesn't sort Keys of Collection

iam sitting here since 9am and breaking my head about this weird stuff.

Im trying to group a collection of entries by the year and month of created_at.

My code looks like this:

Auth::user()
   ->entries()
   ->get()
   ->sortByDesc('created_at')
   ->groupBy(
      [   
         function($item) {
            return $item->created_at->year;
         },
         function($item) {
            return $item->created_at->month;
         },

      ]
   )
   ->sortKeysDesc()

But all I get is this:

{
    "2018": {
        "12": [
            {
                "id": 8,
                "user_id": 1,
                "name": "et",
                "value": -8.6,
                "created_at": "2018-12-31 20:59:06",
                "updated_at": "2019-02-03 19:50:20"
            },
            …

        ]
    },
    "2019": {
        "1": [
            {
                "id": 42,
                "user_id": 1,
                "name": "non",
                "value": -3.5,
                "created_at": "2019-01-31 05:36:01",
                "updated_at": "2019-02-03 19:50:20"
            },
            …

        ]
    }
}

My target is to get the latest years at first. 2019, 2018… As you can see, the sortKeysDesc() doesn't do anything at all. Yes – i tried sortKeys() as well -> same result.

Whats wrong with my approach?
Thank you.

via Chebli Mohamed