When dragging a space icon to reorder it in your list of spaces, a space cannot be dragged to be the last space.

I believe this is because in order for a space to move down, it needs to be dragged to below its current position. When you reach the last position and want to move down further, the mouse gets dragged over the '+' sign (for adding new space) and the trigger to move the space simply doesn't happen.

A gap between the last space and the '+' will solve this issue.