Flag/Reject Shortcuts Not Working Properly

"It's very annoying to me, in my workflow, I use second monitor in Grid mode to browse photos and use the main screen to do the edit, I can't mark multiple photos as rejected in second monitor Grid mode by X."

Two less-convenient workarounds:

- Select multiple photos, right-click them, and do Set Flag > Rejected.

- Select multiple photos, make sure you're in Library Loupe view, and do the menu command Metadata > Enable Auto Sync (or click the on/off button to the left of Sync in the lower-right corner).  Now any change you make to photo metadata in Library Loupe view will be applied to all the selected photos. I don't recommend this option, since it's too easy to forget that you've left it on.

LR is highly inconsistent with how it deals with multiple selected photos in Loupe view ("inconsistent" is being polite -- it's a pigsty).  You could post a feature request to change the behavior or add an option to change it.  Adobe wants all product suggestions posted in the official feedback forum:
https://www.lightroomqueen.com/send-bug-report-feature-request-adobe/

Product developers read everything posted there, but rarely participate here, which is primarily a user-to-user forum. (Safari users, uncheck the option Safari > Preferences > Privacy > Prevent Cross-Site Tracking or use Chrome or Firefox.)

However, with this particular feature request, I think it's highly unlikely Adobe will ever act on it. Over the years, employees have consistently defended the current behavior.

[Use the blue reply button under the first post to ensure replies sort properly.]

See: https://helpx.adobe.com/lightroom-classic/help/flag-label-rate-photos.html

and note down under  Flag or reject photos:

• Select one or more photos in the Grid view, or select a single photo in the Filmstrip in Loupe, Compare, or Survey view. Then, choose Photo > Set Flag and choose the flag you want. If more than one photo is selected in the Filmstrip in Loupe, Compare, or Survey view, the flag is applied only to the active photo.

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It is also doing the same for ratings and color coding images.