Hi @sKuijers
The acf_form_head function is only intended to be used in a front end template. It can’t be used in the backend in the same way.
For ACF to work, you will need to use the acf/input/head, acf/input/enqueue_scripts actions. Perhaps you could look in the core/controllers/input.php file to see how ACF adds the necessary code to the post page?
Thanks
E