Sinon stub won't stub the function

I have the following abstract class which uses the "fs-extra" library:

import { ClassConstructor, instanceToPlain, plainToClass } from 'class-transformer';
import { validateOrReject } from 'class-validator';
import fs from 'fs-extra';

import getLog from '../../helpers/logger';

export default abstract class AppConfigService<T extends object> {
    public async setConfig(config: T) {
        const logger = getLog(`${AppConfigService.name}/${this.setConfig.name}`);

        logger.info(`About to set app-config file with configuration:\n${JSON.stringify(config, null, 2)}`);

        const currentFileObject = instanceToPlain(this.fileObject);

        logger.info(`Current app-config configuration is:\n${JSON.stringify(currentFileObject, null, 2)}`);

        const validatedConfig = await this.validatedConfig({ ...currentFileObject, ...config });

        logger.info('Provided app-config configuration is valid');

        const filePath = await this.getFilePath();

        logger.info(`Got app-config file path: "${filePath}""`);

        await fs.outputJson(filePath, validatedConfig);

        this.fileObject = validatedConfig;

        logger.info('Successfully set app-config provided configuration');
    }
}

This is only snippet as I don't find the rest of the class relevant.

Then I have the following class:

import path from 'path';

import AppConfigService from '../../services/app-config';
import ConstantsModule from '../constants';
import { UserAppConfigDto } from './classes/user-app-config.dto';
import { APP_CONFIG_FILE_NAME } from './models/user-app-config';

export default class UserAppConfigService extends AppConfigService<UserAppConfigDto> {
    protected classConstructorRef = UserAppConfigDto;

    protected async getFilePath() {
        const userConfigFolderPath = await ConstantsModule.service.getUserConfigFolder();
        const appConfigFilePath = path.join(userConfigFolderPath, APP_CONFIG_FILE_NAME);

        return appConfigFilePath;
    }
}

And then I try to test the last class:

import { doesNotReject, rejects } from 'assert';

import sinon from 'sinon';
import { expect } from 'chai';
import fs from 'fs-extra';

import UserAppConfigModule from '../../app/modules/user-app-config';
import UserAppConfigService from '../../app/modules/user-app-config/user-app-config.service';

describe('[modules/user-app-config.module.ts]', () => {
    const sandbox = sinon.createSandbox();

    afterEach(sandbox.restore);

    describe('setConfig()', () => {
        it('should throws when "fs-extra/outputJson" rejects', async () => {
            sandbox.stub(fs, 'outputJson').rejects();

            await rejects(UserAppConfigModule.service.setConfig({}));
        });

        it('should succeed when "fs-extra/outputJson" resolves', async () => {
            sandbox.stub(fs, 'outputJson').resolves();

            await doesNotReject(UserAppConfigModule.service.setConfig({}));
        });
    });

});

But for some reason sinon does not stub fs-extra, the function outputJson. Why is that? I suspect the fs-extra/outputJson is not stubbed because of any context issues? My first test fails because the function wasn't stubbed