$matching on a positional path only seems to work as expected within a $facet if there are no subsequent stages. $count, $project, $group all behave as though they receive zero input documents, even though results are correctly returned when no subsequent stage is provided.

$sort, however, seems to function correctly.

original description

The below script leads to an empty scoreRank:

db.test.drop();

db.test.insertMany([{
   "_id": 12345,
   "quizzes": [
     { "score": 100 }
   ]
  },
  {
    "_id": 789,
    "quizzes": [
      { "score": 200 }
    ]
  }
]);

const res = db.test.aggregate([
  {
    $facet: {
      scoreRank: [
        { $match: { 'quizzes.0.score': { $gt: 0 } } },
        { $count: 'count' }
      ]
    }
  }
]).toArray();

print(JSON.stringify(res, null, '  '));

I get the below output:

[
  {
    "scoreRank": []
  }
]

If you replace 'quizzes.0.score' with 'quizzes.score', move $match out of $facet, or move the entire scoreRank pipeline out of $facet I get the expected result:

[
  {
    "scoreRank": [
      {
        "count": 2
      }
    ]
  }
]