I am using a plugin named Mailster to create newsletters. It has a custom post type named 'newsletter' and I am trying to use the Meta Box Builder plugin to add a custom field.
It works as intended but with an odd bug. The custom field will not display the field initially on the post edit screen. However, if I choose uncheck and recheck the box under Screen Options it will then appear and works as expected. However this needs to be done every time I create or edit a newsletter.
I'm trying to figure if it's Meta Box or Mailster that's causing the issue. Like I said the custom field works as expected after unchecking/rechecking in Screen Options but my client is non-technical so I'm trying to keep it as straightforward as possible.