The only thing that I can really give you is that ACF stores and array of attachment IDs (post IDs) of the images in the gallery.
ACF does not necessary associate these images with the post that the gallery field is attached to. This is a function of WP.
If you are editing a post when you upload the images then the images are associated with the post (uploaded to) basically, this means that the parent of the attachment is the post you are editing.
If you upload images to media and then add them to a gallery then the attachment is not made by ACF.
Images can only be “attached” to a single post, they can only have 1 parent.
Not sure if this information will help you or not.
