I have that problem with a lot of scenes. Almost always I can fix it by undo'ing the opening of the scene, and then hitting the forward to reopen it. Some scenes require you to do this like 3-4 times before they load correctly.
That's not a bug, just a graphics artifact. If you check the angular velocity of the circle it's spinning very fast, and it just happens to match the refresh rate of the program so it appears it isn't moving.
If you turn the strength of the motor down to 0 and wait about 30 - 40 seconds the motor will slow down to a stop as you'd expect.
