I have had that happen when using a "shape tween" but never a motion tween. If
the bar on the timeline is a green color it's a shape tween, if the bar is blue
it's a motion tween. If it's a shape tween by accident you can select the key
frames, right click "remove tween" then make a new motion tween.

If it is blue colored motion tween doing that, it could be a program error.
Try clicking your object and copying it, deleting the strange motion tween and
making a new tween.