It seems to be only the second/middle slot for me. I know I'm bumping a really old message here, but this is still a bug, and while I have Calamity installed, it is not enabled and has never been active at all at the same time as this mod, but I've had the same issue still today, many months after your message. It is especially annoying to have to farm cores to make your prism back every time you play simply because the mod just makes them vanish for no reason.