@steve.dower nailed it: JFrog is performing a HTTP 302 redirect to S3, which is probably confusing pip.
Could you share a bit more of your setup? I’m already familiar with simpleindex
(see Advice to avoid `--extra-index-url` to install private packages from GitLab CI - #8 by astrojuanlu) so I will follow your advice and give it a try for this problem.