I did some experiment locally. At very first I thought it's the image compression issue, since the issue is also obvious while rendered out images as JPG and PNG, there's no image compression quality settings in MRQ and the image size rendered out by MRQ is much smaller than the ones rendered by Sequencer legacy render. But after trying bmp the issue was still exist. Also the banding issue is vary in different color space. So I assume there might be something different in terms of Color converting while rendering out those images in MRQ
The banding issue is not visible in Editor viewport neither
This issue is more obvious in Dark gradient pixels, setting up such environment first is a easy way to reproduce it
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-122226 in the post.