S3- "not permitted" errors on public link (file drop)

For some odd reason, when I use the “public/file drop” option to share a folder on an external source (Amazon S3), and a recipient uploads… they are presented with a “could not upload” option, and the above permissions error in the Nextcloud log (above) is observed. It’s important to note that the file actually does upload, and is downloadable/playable, which makes this error even more confusing.

Any ideas as to how I can fix this? Can it be fixed, or should I just be sharing local folders, instead?