So I was having this problem on Premiere Pro, and google led me to this thread so I’m going to post my solution here, just in case it saves some poor soul an hour of troubleshooting.
Apparently PrPro didn;t like that my bottom layer footage had a lumetri color effect and my second (top) layer of footage had opacity at 30%. This of all things caused the feathered mask on my second layer to render out with terrible banding. I removed those attributes and the banding ended.
My workaround: Instead of changing the opacity of my footage, I added a color matte similar to the background color above both layers, and changed it’s opacity instead. Hope this could help someone.