Mirror Modifier broken by undo's

Hi guys. I have been trying to make a mesh of a face, using the mirror modifier. However when I make the a mistake and pres ctrl-z to undo, sometimes half of the mesh stops being mirrored. I have no idea why this happens. Is this a bug? Or is some aspect of blender escaping me?

Edges won’t show if they are on a planar surface

Go out of edit mode
Change out of wireframe view
Enable the ‘Apply Modifier to editing cage during edit mode’ mode on the top row of icons in the mirror modifier panel