Code to reproduce issue

require('dotenv').config();

const mongoose = require('mongoose');

mongoose.connect(process.env.URL_SERVER_MONGODB,  { useNewUrlParser: true }, (err, db) => {
    // There was an error connecting to the database
    if (err) console.log(err);

    db.createCollection('players', {
        validator: {
            $jsonSchema: {
                bsonType: "object",
                additionalProperties : false,
                required: ["_id", "friend_code", "display_name"],
                properties: {
                    _id:          { bsonType: "string", description: "must be a string" },
                    friend_code:  { bsonType: "string", pattern: "^[0-9]{4}-[0-9]{4}-[0-9]{4}$", description: "must be a string" },
                    display_name: { bsonType: "string", description: "must be a string" }
                }
        }
    } }, (err, response) => { });

    let players = db.collection('players');

    players.createIndex({ "friend_code" : 1 }, { unique: true } );

    players.insertOne( {_id: "3", friend_code: "a", display_name: "Welcius"}, (err, response) => { console.log(err); });

    db.close();

});