Thanks for reaching out via the Apple Support Communities, Thierry,
If I understand your post, iCloud storage indicates it's taking up roughly 28 GBs of storage, even though you cannot find the data that is supposedly using the storage. I'm happy to help!
It sounds like you've already done some great steps by verifying what is shown at iCloud.com, in the Recently Deleted folders, and in the various apps on your iPhone. If you also use a Mac computer with iCloud, I'd suggest next making sure that there are no files in the Trash as well.
Whether you use a Mac or not, I would also recommend deleting files under Settings > Advanced > Restore Files directly from iCloud.com using a web browser on a computer. You may want to archive or copy files from iCloud before completing this step if there is anything you do not want to lose. If this computer is a Mac, restart it once you've removed the files: How to turn your Mac on or off - Apple Support.
If the issue remains unresolved after this, please get in touch with Apple Support directly for additional assistance: Contact Apple Support.