The Delete key will sometimes delete a face region from a previously viewed photo, that is not currently selected
While working through recently added photos, if you tag a face on a photo, then move on to the next photo, and try to delete that photo, Lightroom will instead appear to do nothing but actually delete the face tag that you added to the previously viewed photo.
Specifically, the steps to reproduce this are:
View a single photo in the Library (Loupe view) Add a face region to the current photo Go to the next photo which does not have any face regions (Ctrl+Right) Attempt to delete that photo by pressing the Delete key
Nothing appears to happen - the Confirm message doesn't appear and the photo isn't deleted
However, if you go back to the previous photo with Ctrl+Left, you'll see that the face region that you just added has been deleted. Lightroom shouldn't do that to a photo that is not visible or selected.