While in the Developer module, I have selected several photos, then hit the Del key, and I am informed that the "selected master photo" in singular will be deleted, and when clicking OK, only one of the selected photos are deleted. When I am in the Library module and do the same, all three photos are selected. Is that a bug or a "feature"? In any case, I suggest this is changed so the experience is the same in both.
I believe that is the way the delete key is supposed to work. If you want to delete multiple images, the way to do it is to go through and mark them for deletion by pressing the letter X on each of those images. After you have selected all of those images then press Ctrl+BckSpc, and you will be given the option to remove them or delete them from disk.
Using the method I have described, if the images are adjacent, if you activate the caps lock, pressing the X will advance the highlight to the next image. If you don't want that image deleted then pressing the spacebar will advance the highlight to the next image without deleting the highlighted image.
Right that is how it works when you select images in the Film strip either in develop or the Library module when only one image is displayed in the main window, loupe view in the Library module. The Primary selected image, the one that is brighter, lighter, gray color is deleted.
If you want to select and delete multiple images do it in the Library module in the GRID view area.
You can also mark image, single or multiple, with the x (Reject) key and then use the Ctrl + Backspace key combo to delete them all. This can be done in either the Library or Develop module in the Film strip area. but you have to select each image then hit the x key.
If you try to select more than one image in the Film strip and then x them only the primary image gets the x, Reject, flag.
That is more than likely because the Grid view is being displayed. If you go to Loupe view only the primary will be deleted.