If I can find some time I will see if I can test what is submitted. My thought is that the current user ID must be passed as part of the ajax request because WP would check to ensure that the user is logged in and has access to the media library.
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We demoed ACF 6.1 Beta during the most recent session of ACF Chat Fridays, highlighting the new ability to regenerate and clear labels, setting the Admin Menu Parent as a slug, and more. Catch the video replay in our latest summary. https://t.co/rHEpPVas64pic.twitter.com/hB1XKTexXi