would you believe me if i found it again-
this time, it wasn't caused by me re-opening the game, i just left the game running for about a minute while i was tabbed out, and the miku-neru fusion appeared on my cursor and deleted neru on the box.
i should also note that it occurred after beating neru ex, if that means anything.
edit:
i managed to make the sprite move to a different location by hovering my cursor somewhere else and waiting for a while (second image). i thought i found a consistent way of replicating the bug until i tried it in other rooms and couldn't get it to work. i even attempted to do the same method of waiting in the neru room to trigger the event, but couldn't get it to work even after leaving it on for nearly an hour.