Cursors should be installed in /usr/share/cursors/xorg-x11/ and not in /usr/share/icons/
In times where xorg-x11 isn't strictly necessary anymore, perhaps the path was changed to a more general location? In any case, I haven't found a problem with the cursors at runtime so far.
This is hard-coded upstream. I guess they are not likely to change it to an X-specific location as wayland migration work is ongoing.