Bootstrap 4 "hidden" class doesn't work

Use d-none, d-sm-none, d-md-none etc.

https://getbootstrap.com/docs/4.0/utilities/display/

To only display for md up (hide for sm down) use:

<div class="d-none d-md-block"></div>

Well, using the display property definitely solved the problem.

In case anyone wonders why the hidden classes didn't work, it's very simple... They no longer exist in version 4 of Bootstrap. That's what you get from watching old tutorial videos.