MongoDB aggregation structure causing some values to return as none

I have written a simple function using mongoDB aggregation pipelines. The issue occurs when attempting to run it. For some reason min_price and max_price both return as none instead of a real value. The individual documents are formatted as so,

  {
    "_id": {
      "$oid": "62afc6584c7476a02643b61f"
    },
    "item_name": "Titanic Super Heavy Leggings §6✪§6✪§6✪§6✪§6✪",
    "auction_id": "516d82192fef4c3aa178b134e75fc0e1",
    "seller": "dae37eba5f594922bc8d79b9a78f8543",
    "seller_profile": "585d9e30f644496cafa0b36b555ef788",
    "buyer": "7a3699ca1eef40929c714362ab35915a",
    "timestamp": {
      "$numberLong": "1655686642260"
    },
    "price": {
      "$numberInt": "1000000"
    },
    "bin": true
  }

Below is the function used to calculate min and max and sorting them by date.

def min_max_volume(item_name):
    db = client.test
    mycol = db["ended"]
    pipeline = [
        {"$match": {"item_name": item_name}},
        {"$project": {
            'datetime': { '$dateToString': { 'format': "%Y-%m-%d", 'date': { "$toDate": "$timestamp"}}},
        }},
        {"$group": {
            '_id': '$datetime',
            'min_price': {'$min': '$price'},
            'max_price': {'$max': '$price'},
        }}
    ]
    results = mycol.aggregate(pipeline)
    for i in results:
        print(i)

The expected output should look like this,

{'_id': '2022-06-20', 'min_price': 345839475, 'max_price': 48534875}
{'_id': '2022-06-21', 'min_price': 456567, 'max_price': 348573945}
{'_id': '2022-06-22', 'min_price': 6486956, 'max_price': 12938291}

but instead does not contain values in the min_price and max_price fields.

{'_id': '2022-06-20', 'min_price': None, 'max_price': None}
{'_id': '2022-06-21', 'min_price': None, 'max_price': None}
{'_id': '2022-06-22', 'min_price': None, 'max_price': None}