The behavior in Loupe view is very confusing when multiple images are selected. Over the years there's been a steady stream of users posting their confusion. Despite myths to the contrary, there is no single unifying principle for this behavior. In a better world, Adobe would implement a simple, easy to remember rule for all commands.
With many of the commands, the keyboard shortcuts and right-clicking on the Loupe image only affect that image (the most selected image). But right-clicking on a filmstrip thumbnail applies to all the selected images (even if you can't see them because they're scrolled off screen). Commands in this category include Paste Settings, Set Flag, Remove From Quick Collection, Remove From Collection, Remove Photo (from folder), Create Virtual Copy, Edit In > Edit In Photoshop, Edit In > Open As Smart Object. But Reset and Proof are exceptions -- they only apply to the most-selected image.
Some commands operate on all selected photos regardless of whether you right-click the Loupe image or the filmstrip: Rotate Left/Right, Photo Merge, Export, Stacking > Group Into Stack, Edit In > Merge To Panorama, Edit In > Merge To HDR, Edit In > Open As Layers.
Add To Quick Collection and Set Flag are even more confusing: When invoked by right-clicking a filmstrip thumbnail, they apply to all selected images. But when invoked by clicking their badge on a thumbnail, they apply only to the most-selected image.
If you have multiple filmstrip thumbnails selected and you right-click an unselected thumbnail, some commands operate on the most-selected image (Reset, Proof, Paste Settings, Paste Settings From Previous), while others operate on the right-clicked image (Auto White Balance, Auto Settings, Convert to B&W).
And at least one command, Metadata Presets > particular-preset, asks you if you want to apply to the most-selected photo or all selected photos, regardless whether you right-click on the Loupe image or the filmstrip.
The Sync button in Develop Loupe applies to the entire selection, while the Paste button applies to the most-selected image only.
The behavior is determined by the mode of the main display. If the main display is in Loupe, then the commands act only on the most-selected photo, even if grid view is visible on the secondary display.
While you can come up with use-cases motivating these different behaviors, it seems arbitrary and certainly most people couldn't remember the rules.