Welcome to the browser nightmare of undo/redo. I will need to double check this, but when you use the context menu to undo, you completely bypass any behaviors that we (or any other editor, text box, etc) implement as the browsers don't actually expose undo/redo events. We write custom handling for keyboard shortcuts and those menu buttons.
As such, there's basically nothing we could do about this (short of killing the context menu, which has all manner of other issues).