In motion Tween the basics are, having a first and a last keyframe which has
the tween in between them. When i change the position of the object a bit in
the last keyframe, the motion tween is ruined and in middle part of the motion
tween the object turns out to be transparent and shows the background colour.
What i want to know is that if the purpose of motion tween is just to transform
an object by position or colour or some other characteristics, then how does
changing the position of the object make a difference to its appearance which
it should not do.:confused;

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.

i have tried what you are asking me to do, and i am pretty sure that it is a motion tween and not a shape tween...