Sequelize Connection issues and unexpected NULL values

Question

I'm having trouble with my Sequelize ORM connection to a MySQL database. I'm receiving a "User is not associated to Bill" error on some requests to the backend server. User and Bill are tables in my Sequelize ORM, and they have a one-to-many relationship. Interestingly, the error disappears when I perform some requests to the server, but appears again when the server establishes a connection to the DB after some period of time.

As a result, some mistakes are happening in my database, such as unexpected NULL values in the BillId.ProductId column.

Here is how I've initialized the connection to the DB in my code:

// bill.model.js
const { DataTypes } = require('sequelize');
const db = require('../../config/db');

const Bill = db.define(
  'Bill',
  {
    // ... Columns definitions
  },
);
Bill.associations = function associations(models) {
  Bill.hasMany(models.BillItem);
  Bill.belongsTo(models.User);
};
module.exports = { Bill };

// config/db.js
const db = new Sequelize(DB_URL, {
  logging: false,
});

(async () => {
  try {
    await db.authenticate();

    const { BillItem } = require('../modules/bill_item/bill_item.model');
    const { Product } = require('../modules/product/product.model');
    const { Bill } = require('../modules/bill/bill.model');
    const { User } = require('../modules/user/user.model');

    await User.associations({ Bill });
    await Bill.associations({ User, BillItem });
    await BillItem.associations({ Product, Bill });
    await Product.associations({ BillItem });

    await db.sync();
  } catch (error) {
    console.error('Failed to connect with the DB: ', error);
  }
})();

module.exports = db;

Can anyone help me identify the issue with my Sequelize ORM connection and why I'm receiving unexpected NULL values in my database?