In Library Loupe view, with multiple photos selected and Auto Sync disabled, the Metadata panel lies to the user by highlighting Selected Photos and displaying a tooltip saying "Show metadata for all selected photos". But the metadata for just the most-selected photo is displayed, and any changes will be made just to that photo:
The selection behavior in Loupe has always been confusing, and this user-interface lie makes it even more so, e.g. see:
The product team says this behavior is "as-designed", which implies there is a design rationale -- what could it be?
The correct behavior is for Loupe to behave just as when one photo is selected in Grid view: Both Target Photo and Selected Photos are grayed out, and the metadata for the most-selected photo is displayed.
When Auto Sync is enabled, the current behavior is sensible. With one photo selected, Target Photo and Selected Photos are grayed out. With multiple photos selected, Target Photo and Selected photos are not grayed out, and Target Photo shows the metadata for the most-selected photo, while Selected Photos shows summary metadata for all the selected photos. The highlighted Auto Sync button clearly indicates that changes will be made to all the selected photos.
Here's a summary of the correct behavior, with red highlighting where the bug is: