TypeError: Cannot read properties of undefined when running combined tests in Express backend with chai and chaiHTTP

I’m working on a couple of tests for a backend created with Express and running with Node. The tests works when I run them separately / individually, but not when I run them together.

npm run test-auth  ==> Ok
npm run test-api   ==> Ok

npm test  ==> Fails

Error message:

1) Authentication
     addUser
       should register a new user:
   TypeError: Cannot read properties of undefined (reading 'execute')
    at Context.<anonymous> (file:///home/k/backend/test/test-authentication.mjs:22:26)
    at process.processImmediate (node:internal/timers:478:21)

The testfiles looks similar, in terms of app, chai.request.execute etc.:

import { use, expect } from 'chai';
import chaiHttp from 'chai-http';
import { app } from '../src/server.mjs';

const chai = use(chaiHttp);

describe('Authentication', function() {

    describe('addUser', function () {
        it('should register a new user', function (done) {
            const email = `${Math.random().toString(36).substring(7)}@disney.com`;
            chai.request.execute(app)
                .post('/graphql')
                .send({
                    query: `
                        mutation {
                            addUser(email: "${email}", password: "donaldduck") {
                                email
                            }
                        }
                    `
                })
                .end((err, res) => {
                    if (err) {
                        console.error(err);
                    }
                    expect(res).to.have.status(200);
                    expect(res).to.be.json;
                    expect(res.body).to.have.property('data');
                    expect(res.body.data).to.have.property('addUser');
                    expect(res.body.data.addUser).to.have.property('email', email);
                    done();
                });
        });
    });
});

My scripts looks like this:

  "scripts": {
    "test": "NODE_ENV=test mocha 'test/test-*.mjs' --experimental-modules --timeout 5000 --exit",
    "test-auth": "NODE_ENV=test mocha test/test-authentication.mjs --experimental-modules --timeout 5000 --exit",
    "test-api": "NODE_ENV=test mocha test/test-api.mjs --experimental-modules --timeout 5000 --exit",
    "start": "node src/server.mjs"
  },

I did try a couple of things, such as adding --flag to test, change chai.request.execute(app) to chai.request(app), etc. and am now stuck, at least if I want the tests to be in separate files.