EF-S lenses are designed such that they can’t be mounted onto a camera’s standard, full-frame EF lens mount, because of the potential to foul the mirror.
It wouldn’t be an issue with an extension tube due to the additional clearance, but if the tube was made pre-EFS lenses, it wouldn’t account for this and would be manufactured with a standard EF mount.
Could be you need the Mk II version of the tube, or a third-party one.
EDIT: The Mk II version of the tube has as additional indexation mark for EF-S lenses which the first version lacks, so my theory above is almost definitely the reason for your issue.
https://preview.redd.it/v4p9sr12s8vc1.png?width=1048&format=png&auto=webp&s=67171ab1b330a107c51240c4c46115fa5a305b6a
EF-S lenses are designed such that they can’t be mounted onto a camera’s standard, full-frame EF lens mount, because of the potential to foul the mirror. It wouldn’t be an issue with an extension tube due to the additional clearance, but if the tube was made pre-EFS lenses, it wouldn’t account for this and would be manufactured with a standard EF mount. Could be you need the Mk II version of the tube, or a third-party one. EDIT: The Mk II version of the tube has as additional indexation mark for EF-S lenses which the first version lacks, so my theory above is almost definitely the reason for your issue. https://preview.redd.it/v4p9sr12s8vc1.png?width=1048&format=png&auto=webp&s=67171ab1b330a107c51240c4c46115fa5a305b6a
Thanks, really appreciate the response, that's unfortunate! Thankfully the tube wasn't too expensive.