Keyboard Shortcut for Editing Photos pix in Preview

The ⌘+Return open the image in the last app that was used to edit a photo in the library. You can setup Preview as an editor by installing this extension for Photos:

Then when in the Edit mode click on the Extensions button

and select Preview as an editor. Then to make it the app that ⌘+Return opens Control (right) - click on the thumbnail of the image and select Preview from the list of editor in the menu:

From then on ⌘+Return will open the select thungnail image in Preview.

When I use command-return on one of my thumbnail pictures in Photos, it opens in Preview, like you say it should. What happens if you just hit Return? Do you go into Photos edit mode?

What operating system are you using-- macOS 14 Sonoma, or another? Do you use Share Albums?

Well, that seems weird. Have you gone through these things?

• Restart the computer (of course)
• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. (Close Photos and Option-click on the Photos Icon to Create New library)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it may fix some funky goings on. Safe Mode is different for different computers, so see this:

Start up your Mac in safe mode - Apple Support 

• Make a new user and see if the same problem recurs with it

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. 

Let us know if anything changes...