According to the ts-jest team, this way (https://github.com/vuejs/vue-jest/blob/v27.0.0-alpha.2/packages/vue3-jest/lib/index.js#L3) of exporting transformer no longer works with Jest 27.

The right way to export transformer is show here: https://github.com/kulshekhar/ts-jest/blob/master/src/index.ts :

import { TsJestTransformer } from './ts-jest-transformer'

export default {
  createTransformer: (): TsJestTransformer => new TsJestTransformer(),
}