Amazon Photos is one of the worst when it comes to reading photo metadata. It will read embedded EXIF “date taken” from some file formats, like JPEG. It definitely does NOT seem to understand XMP sidecar files at all - although it CAN (sometimes) read the original Date Taken embedded in (some) proprietary RAW formats (example: Canon .CR2). But any metadata edits done on RAW files in Mylio will of course be written only to the XMP sidecar file (remember Mylio never touches your RAW files) - so of course Amazon will completely ignore those.
For JPEG, TIFF, and DNG (which do support embedded metadata) you’ll probably have to tell Mylio to first save the metadata to the file (
Photo -> Save Metadata to File) and then export & upload them to Amazon.
As a secondary issue, you’ve probably already discovered that syncing from Mylio to Amazon results in an almost completely unusable mess. No folder structure, long random file names, duplicate files (thumbs, previews, originals), etc. It’s important to keep in mind that Mylio’s “cloud sync” options are intended ONLY for Mylio backup/restore/sync usage - NOT as a way to publish your photos for easy browsing or viewing.