Can't access array element in mongoose

I have documents that have metadata.

{
    "_id": "6158d1e84cec41f47c8fde51",
    "metadata": [
        {
            "source": [{info: "xxxxxx", timestamp: 0}, {info: "xxxyyy", timestamp: 1}],
            "email": "xxx@xxx.xxx"
        },
        {
            "source": [{info: "yyyyyy", timestamp: 0}, {info: "yyyxxx", timestamp: 1}],
            "email": "yyy@yyy.yyy"
        }
    ]
}

Starting with the sixth version of mongoose, an empty array was returned. The reason for this was that the condition below stopped working.

"metadata.source": {
    "$exists": true,
    "$not": {
        "$size": 0
        }
    }

I wrote an alternative version that reduces the projection and it seems to work like that.

"metadata": {
    "$elemMatch": {
        "info": {
            "$exists": true,
            "$not": {
                "$size": 0
            }
        }
    }
}

But I wanted to know what was wrong with the first option, since everything works in mongodb compass and mongoose below the sixth version.