The preview works for the main administrator role but not other roles (e.g. Editor and Contributor).
My solution was to go to /wp-admin/users.php?page=roles&action=edit&role=editor and click Fields and then grant manage_options.
I then logged in as an Editor and hid the now exposed custom fields but editing a page, clicking on “Screen Options” and unchecking “Custom Fields”.
This is not ideal because it opens up options that other roles really shouldn’t see.
