Magnetic Mask Issue Final Cut Pro

Hello community,

I'm trying to use the magnetic mask and replace the background and put the subject over another background image. I'm getting horrible flickering around the subject mainly in the right shoulder area (just like with the old green screen). I've included a video sample of the mask applied and the original video clip. Of course all of the tutorials I watch on YouTube it works perfectly. :(

Thank you in advance for your help!

https://youtu.be/jaJBRmIin_Q