Hi,
My bad. I found this support ticket https://support.advancedcustomfields.com/forums/topic/acf-pro-5-6-0-unable-to-display-regular-custom-fields-metabox-on-cpts/.
It can be fixed with the following filter:
add_filter( 'acf/settings/remove_wp_meta_box', '__return_false' );
So the ticket can be closed.
