I had to hit the brakes on this one… you positioned the 3D cursor for the purpose of adding the mirror modifier by creating a new vertex. That feels a little convoluted. Is there any other way to achieve this? Perhaps it’s just a limitation of Blender, and maybe if the cursor was selectable (weird, yeah), it could make sense. (In fact, why not make the cursor selectable!!!…. with a hotkey to override that ability :))
Maybe it’s a well-known trick… but I paused… rewound… did some searching, scratched my head, and then the “ahhhhh.. I see” sunk in.