I’m sorry, but I don’t know why the ACF call wouldn’t work. The POI layout runs within that same template, so it should behave identically to a call from the template.
Does it work if you use a regular (non-ACF) custom field? Perhaps you can have ACF save the value to a custom field you can read with get_post_meta() instead.
Also, could you post your entire template file or email it to me?