Mongodb aggregate $group not working in my case

Working with Data
1

Hello there:

I want to pull the latest chatBucket between two users. There are 51 messages in each chatBucket. “recent_t” is the timestamp column stored as a number in the database. I use an aggregation query with “$group,” which returns a different result at different times. According to my research, $group is causing the issues. When I remove the $group, all buckets appear. I think there is something wrong with the codes. I would appreciate any help you can give me. Below is the code.

ChatBucket.aggregate(
  [
    { $match: { unames: { $in: [uname] } } },
    {
      $project: {
        unames: 1,
        recent_t: 1,
        msg: { $slice: ["$messages", -1] },
      },
    },
    {
      $group: {
        _id: "$unames",
        time: { $min: "$recent_t" },
        msg: { $last: "$msg" },
      },
    },
    { $unwind: "$_id" },
    { $match: { _id: { $ne: uname } } },
    { $skip: skip },
    { $limit: 5 },
    { $sort: { time: -1 } },
  ],
  async (e, data) => {
    if (e) {
      reject(e);
    }
    resolve(data);
  }
);