Presumably to be a bug, it actually has quit this mode, just show the drawing line.
I analyze because set selection draw, it ignores Draw grid, so hard to just your starting point on the Draw grid, when you finish drawing the polygon when not in use Close polygon command, it is difficult to ensure that the end point is just at the starting point on, so the graphics will not be closed, the command can not be completed, then you then switch placment or other mode will be such a phenomenon.