Interesting problem. I am using mongoose in an app and have not seen this issue. See schema below:

const mongoose = require("mongoose");
const Schema = mongoose.Schema;

const schema = new Schema({
  createdBy: Object,
  created: { type: Date, default: Date.now },
  matchName: { type: String, required: true },
  matchType: { type: String, required: true },
  matchActive: { type: Boolean, default: false },
  matchPrivate: { type: Boolean, default: true },
  matchTime: { type: String, required: true },
  matchDate: { type: String, requried: true },
  matchPassword: String,
  matchMembers: [],
  course: Object,
  subCourse: Object,
  par: { type: Number, required: true },
  numberOfHoles: { type: Number, required: true },
  frontBackNine: { type: String, required: true, default: null },
  completed: { type: Boolean, default: false },
  compeletedOn: { type: Date },
  winner: { type: String },
  startingHole: { type: Number, default: 1 },
  scorecardSettings: { type: String, required: true },
  tournamentGroups: { type: Array, default: [] },
});

module.exports = mongoose.model("Matches", schema);

However I am not using moment.js.

Two thoughts here:

1. try removing commenting out moment.js code and use:

Date.now

in place of the moment formatted date. This is just to see if moment is causing the issue or not and will not actual give you a solution

2. How are you creating the collection and document? Are you creating a document when the server is started or when the user uploads the file?

Think about when that variable is evaluated and assigned; is when the schema is created, that happens just one time. In any case, you should assign that value at the time when you save the record, assign the value as any other.

But, moongose already has an option to assign timestamps to records when are created and updated. See it here: https://mongoosejs.com/docs/guide.html#timestamps

Your code should look like:

const mongoose = require("mongoose");
const Schema = mongoose.Schema;

const codeSchema = new Schema({
    code: {
      type: String
    },
    createdAt: {
      type: String,
      default: formattedDate
    },
  },
  {
    collection: "codes",
    timestamps: true
  }
);

module.exports = mongoose.model("Code", codeSchema);

About the format, I highlight recommend take care of that after at the final point when you use the data.

You are assigning a predefined value, that's why it is not working, you should create a function and assign the function as a default value to get the value when the document.

const mongoose = require("mongoose");
const Schema = mongoose.Schema;
const moment = require("moment");

var createdAt = function(){
    var d = new Date();
    var formattedDate = moment(d).format("MM-DD-YYYY, h:mm:ss a");
    return formattedDate;
};

let codeSchema = new Schema({
    code: {
      type: String
    },
    createdAt: {
      type: String,
      default: createdAt
    },
  },
  {
    collection: "codes"
  }
);
module.exports = mongoose.model("Code", codeSchema);