This really is not to do with ACF, it is due to your limiting of seeing media files to the person that uploaded them.
The the database description for WP here https://codex.wordpress.org/Database_Description
An image, or other type of file is saved as an “attachment” post type in the posts table. When you limit a user to see only their media you are actually limiting them to see only their own post of this post type by the “post_author” column in the database. This column can only have one value and that value is the person that uploaded it.
In order to allow another user to see these media files you would also need to duplicate all of the “attachment” “posts” and apply the new user ID to them. This would also mean duplicating the actual media files.
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