Group stage in aggregation is being skipped

[Stephen_Lynx]

I don’t know if this is relevant, but this aggregation is performed immediately after another aggregation on the same collection.

 files.aggregate([ {
    $match : {
      'metadata.type' : 'captcha',
      'metadata.expiration' : {
        $lte : new Date()
      }
    }
  }, {
    $group : {
      _id : 0,
      files : {
        $push : '$filename'
      }
    }