Alpha video - masked edge visible


Hoping someone can help with an issue with alpha video within Studio.

I have followed the preparing and embedding alpha video tutorials but after exporting I can still see what looks like some of the mask either side of the video (see pic). I think the issue is due to having a video which is 2160 x 1920 (as the original video is portrait 1080 x 1920) however I’m struggling to find a solution.



This issue was solved by exporting a video which has a much larger width with plenty of empty space horizontally.
The original video size was portrait 1080 x 1920, this was then doubled horizontally to 2160 x 1920 to allow space for the mask layer side by side. Finally the horizontal size was doubled again to 4320 x 1920 to provide some empty space in between the original video and the mask video.