Hi there,
This is a really good question and I’ve experienced the same with an upgrade from from Dawn 4.0 to 6.0.2.
It would seem as it there is some over-compression that is having this effect and it could do with being resolved (the image banner element is almost unusable).
The less than ideal solution is to use the slideshow feature within the customiser and not the image banner - for whatever reason, the image compression is not as pronounced and looks a damm sight better!