Apple Photos not saving metadata

Photos does not export its indexed metadata. And, as far as I can tell, Photos doesn't edit EXIF data.

If the photo has EXIF data, exporting the "original" will include the EXIF data from the original.

Your Scanner would have to generate the EXIF data when it creates the scan file.

If you want to embed EXIF data in the scans you make, use an EXIF editing tool. Photos isn't one.

The latest version of Photos, 5.0 in Catalina, does not transfer that data, that I can find. However, older versions of Photos did this. In fact, photos that were adjusted in previous versions will still export with the adjusted dates, just not any changed with the new version. This is a serious bug in the new version and I just now turned in a bug report at feedback. Here's what I wrote then:

"With latest Photos, if I adjust the date and time of a photo (such as scanned photo) and then export that photo, the exported photo does not have the adjusted Capture Date. If I import the photo into a different photo library, the capture date is the date scanned rather than the adjusted date. I'm not writing about the file date of the export, but rather the stored capture date.

Older versions of Photos would create exportable capture dates. In fact, in the new Photos, if I export (by dragging) a photo created in an earlier version, the adjusted date is exported too.

This creates a problem in that all the work done changing the dates in Photos is trapped within that one library. I cannot transfer the files to another library or give to another person without losing the adjusted date.

Photos doesn't store anything in EXIF data (image metadata). It will display EXIF data, but it doesn't store any of the info in the EXIF data.

If you want to alter the EXIF data, you would need a dedicated tool to do that. I'm not sure it makes any sense to edit EXIF data of a scan because it isn't representative of the captured image in a scan.