Jest genMockFromModule of Bull - can't get mocked function result

Environment:

• NodeJS 12.16.1
• Jest 25.2.4
• Bull 3.13.0

Files:

bull.js - (jest mock class)

'use strict'

const Queue = jest.genMockFromModule('bull')

Queue.prototype.count = jest.genMockFn()
Queue.prototype.count.mockImplementation(() => Promise.resolve(5))

Queue.prototype.add = jest.genMockFn()
Queue.prototype.add.mockImplementation(data => Promise.resolve(null))

Queue.prototype.process = jest.genMockFn()
Queue.prototype.process.mockImplementation(fn => fn('process result1'))

module.exports = Queue

MyClass.js

'use strict'

const Queue = require('bull')

class MyClass {
  constructor() {
    this.queue = null
  }

  async start() {
    this.queue = new Queue('queueName')
    await this.queue.count()
    await this.queue.add({ foo: 'bar' })

    this.queue.process(job => {
      console.log(job)
    })
  }
}

module.exports = MyClass

MyClass.test.js

/* eslint-disable no-undef */

'use strict'

jest.mock('bull')

const MyClass = require('../../src/MyClass')

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms))

test('count functions', async () => {
  expect.assertions(9)

  const instance = new MyClass()
  await instance.start()

  await sleep(1000)

  // called 1 time - MyClass.js => await this.queue.count()
  expect(instance.queue.count.mock.calls.length).toBe(1)
  expect(instance.queue.count.mock.calls[0]).toEqual([])

  // called 1 time - MyClass.js => await this.queue.add({ foo: 'bar' })
  expect(instance.queue.add.mock.calls.length).toBe(1)
  expect(instance.queue.add.mock.calls[0]).toEqual([{ foo: 'bar' }])

  // called 1 time - MyClass.js => this.queue.process(...)
  expect(instance.queue.process.mock.calls.length).toBe(1)
  expect(instance.queue.process.mock.calls[0][0]).toBeInstanceOf(Function)

  // here the issues => they all return undefined, but in bull.js every function returns something
  console.log(instance.queue.count.mock.results)
  // set .not.toBeDefined() but they shuold have some value
  expect(instance.queue.count.mock.results[0].value).not.toBeDefined()
  expect(instance.queue.add.mock.results[0].value).not.toBeDefined()
  expect(instance.queue.process.mock.results[0].value).not.toBeDefined()
})

As commented, the mock.results all return undefined, but in bull.js every function returns something.

How can i get the results of the mocked functions?

Also, i have a Class that read from a queue, make some elaborations and add the result in another queue. How can i setup the mock in order to let it read the desired value to elaborate? Check what has been queued as Class result it's ok cause i can read the input of the queue.add() mocked function.