The red square movieClip instance is located in a masked layer without tween. There is only 1 keyframe in that layer. At frame 1, I rotate the red square clip 45 degree.
The first example is the clip masked by shape tween. The shape Tween contains 3 keyframes, tweening from square to circle and then square. See the resetting and reloading of red square. It seems that the masked square automatically reload and reset to a new instance in the next tween segment.
Now, we try motion tween. The mask consists of 3 keyframes, a square move to right and tweening back to left. The masked square does not reload or reset. It remains in 45 degree rotation through these 2 tween segments.