gistfile1.txt

/*
Works
db.getCollection('users').find( {
     _id: {
    $in: [
         ObjectId("5ba8633c5e01bb5453782a12"),
         ObjectId("5ba8802b5e01bb25e335c352"),
         ObjectId("5d71cb23ab360c12ba1f0df2"),
         ObjectId("5dc0e12eab360c77e940b0c2"),
         ObjectId("5c92e827ab360c2ee837dae2"),
         ObjectId("5ba868e45e01bb5e9d51b6f3"),
        "0050I000007caxBQAQ"
         ]
         }
})
Using only string fails:

db.getCollection('users').find( {
     _id: {
    $in: [
         "5ba8633c5e01bb5453782a12",
         "5ba8802b5e01bb25e335c352",
         "5d71cb23ab360c12ba1f0df2",
         "5dc0e12eab360c77e940b0c2",
         "5c92e827ab360c2ee837dae2",
         "5ba868e45e01bb5e9d51b6f3",
        "0050I000007caxBQAQ"
         ]
         }
*/

        $res = $this->createQueryBuilder()
            ->field('id')->in($ids)
            ->getQuery()
            ->execute()
            ->toArray(); // Failed

        $newIds = [];
        foreach ($ids as $id) {
            $newIds[$i] = strlen($id)<24 ? $id : new ObjectId($id);
        }
        return $this->createQueryBuilder()
            ->field('id')->in($newIds)
            ->getQuery()
            ->execute()
            ->toArray();  // works but it was not necessary last time we talked about it (strategy NONE for _id)

/*
  "query" => array:1 [▼
    "_id" => array:1 [▼
      "$in" => array:7 [▼
        0 => "5c92e827ab360c2ee837dae2"
        1 => "5ba8633c5e01bb5453782a12"
        2 => "5ba8802b5e01bb25e335c352"
        3 => "5d71cb23ab360c12ba1f0df2"
        4 => "5dc0e12eab360c77e940b0c2"
        5 => "5c92e827ab360c2ee837dae2"
        6 => "5ba868e45e01bb5e9d51b6f3"
      ]
    ]
  ]
*/