In PhotoLibrary …
Select a file (or multiple) in the right pane
Drag the selected file(s) to another folder in the left pane
While dragging, the file icon(s) shrink and condense into one, and a plus-sign appears
Hover over the target directory, release the mouse
Quite often, PL will leave behind the icon(s) of the moved file(s) in the source directory. Jumping out and back will usually correct that. But this time, I noticed the files were still there, and this was confirmed by terminal, finder, and another attempt at a move. The second attempt actually had PL prompt me “Do you want to replace?”
After playing with it some more, it really seems to happen most often with multiple files selected.