Hi @Supawat_Vitoorapakor sorry I didn’t realised you asking for drop shadow and this effect is baked into video here only how you may solve problem with standard drop shadow.
EDIT: I have now copied your video to my tests project to see what the video is look like and you have baked in the color it self. Now I know what you trying to achieve but as I never used video this way I’m afraid that I have no solution for you.
As your problem is not related to WF I have find something you can try and further investigate on your own. It is an old article but it may clear some aspects why this happening and how you may or not to solve it. You can find some other articles that have similar content on web.