I don’t have this issue at all.
I have a premium WordPress plugin that creates a custom post type, images from wherever can be placed there, and published. Doesn’t matter if the images aren’t attached to the post. This is the main reason I started using ACF and the gallery add-on, was to free my users from this, as well as having to do a feckin regex to just to pull the ID”s to get a correct gallery order when fetching attachments.