I had the same problem. I used var_dump to see what ACF was returning and it was just a numerical string. So then I used a WordPress function wp_get_attachment_url($my_variable) and that returned the correct URL.
I inherited a fairly complex site from a previous developer. It has a custom post type with accompanying templates and I have selected those to be displayed for the field group, but no luck in the admin area. I am curious if there is a way for this admin area selection to be overridden by a modification to the JSON file that accompanies the field group, and if so, where I would look (aside from the JSON file) to find that control. I’ve never seen this behavior on an ACF site before, so it does not appear to be a bug. I’ve developed and supported hundreds of ACF sites.
