Bug in Mix ?

Say in my resources/js/app.js I add a require for sweetalert :

window._ = require('lodash');
window.Popper = require('popper.js').default;
window.swal = require('sweetalert2');

So I launch npm run dev. My sweetalert is not loaded. But now if i change a bit :

window._ = require('lodash');
window.swal = require('sweetalert2');
window.Popper = require('popper.js').default;

It works !

I have tried with other libraries with same result...

Any idea ?

Thanks