Find query on entity with null property value triggers an update query after flushing when forceUndefined is set to true

[JustLitio]

Previous issues related to this one

#4262
#4266

Describe the bug

If I retrieve Entity A and a nullable property has a null value, an update query on Entity A gets triggered after flushing. Seemlingly, this is because of using forceUndefined: true, and the nullable property gets converted to undefined on entity retrieval, and that gets registered as a change that has to be persisted.

This was noted as fixed in v5.7.2, but now it's happening again

To reproduce

import {
  Entity,
  PrimaryKey,
  Property,
  SimpleLogger,
} from '@mikro-orm/core';
import { MikroORM } from '@mikro-orm/postgresql';
import { mockLogger } from '../../helpers';

@Entity()
export class A {

  @PrimaryKey({ type: 'number' })
  id!: number;

  @Property({
    length: 3,
    name: 'created_at',
    nullable: true,
    type: 'datetime',
  })
  createdAt?: Date;

  @Property({ length: 64, name: 'name', type: 'varchar' })
  name!: string;

}

describe('GH issue 4412', () => {
  let orm: MikroORM;

  beforeAll(async () => {
    orm = await MikroORM.init({
      dbName: ':memory:',
      entities: [A],
      forceUndefined: true,
      loggerFactory: options => new SimpleLogger(options),
    });

    await orm.schema.refreshDatabase();
  });

  afterAll(() => orm.close(true));

  test('update triggered after findOne with forceUndefined true', async () => {
    const mock = mockLogger(orm);
    let a: A | null = orm.em.create(A, {
      id: 1,
      name: 'a',
    });

    await orm.em.persistAndFlush(a);

    a = await orm.em.findOne(A, { name: 'a' });

    await orm.em.flush();

    expect(mock.mock.calls).toEqual([
      ['[query] begin'],
      ["[query] insert into \"a\" (\"id\", \"name\") values (1, 'a') returning \"id\""],
      ['[query] commit'],
      ["[query] select \"a0\".* from \"a\" as \"a0\" where \"a0\".\"name\" = 'a' limit 1"],
      ['[query] begin'],
      ['[query] update "a" set "created_at" = NULL where "id" = 1'],
      ['[query] commit'],
    ]);
  });
});

Versions

Dependency	Version
node	v18.16.0
typescript	5.0.4
mikro-orm	5.7.10
driver	postgresql