Photos App Issue!

Photos may have opened another library than you had been using when you edited the photos, perhaps an older version on a backup volume.

You may want to quit photos, connect all recently used disks, then launch Photos again and check the other libraries from the Library Chooser dialog.

How are you accessing the editing by Photoshop Elements? Are you selecting a photo in your library and using one of these menus to open PSE and edit the photo:

or this menu when in the Edit mode:

If it's either of these methods you must flatten the edited image and save it with the exact same file name and extension or Photos will not know it's been edited.

What version of Photos? What version of the OS?

Is this just the thumbnails in the Library view, or also at full size view?

As a professional photographer I can'r risk losing hours and hours of work. I'm exploring other photo apps for storage and organization....

I strongly agree. Photos is a consumer level app and not appropriate to a professional setting. There is a good range of professional apps available for the Mac including (but not limited to) Lightroom Classic, On1 Photo Raw, Exposure x7, CaptureOne...

It is the industry standard pretty much. On the upside there are loads upon loads of online videos and help resources, Check out YouTube...