Ask about transparent bottom webm format video to obtain the cover image is the original problem

[misTiger]

First, I deleted the background of the original video through video matting and got the webm format video without background. However, after I obtained the cover of the video, I still gave the corresponding cover of the original video. You can have a look at the screenshot I provided. Looking forward to your reply.
This is the original video:

This is the video with the background cleaned out:

This is the cover picture I obtained. I think it should be without background. What do I need to do now?