Quote HOW TO AVOID BORDER´S MASKING VIBRATIONS. See enclosed file

Andres

I found that the background (behind the person) of your video is shot in a relatively messy scene.

It is expected that the mask area you selected on the person will work in vibration because of the motion nature of the video.

The professional shooting setup for this kind of background removal and keying scenario will use green curtain at the scene, and then use chroma key feature to key the background out in post-production phase.

I'm afraid there is not much you can do to remove the background in a better way for the video you currently use.