Your frame comparisons are very helpful. Can you make them PNGs or at least try to alleviate some of that macroblocking in the blacks?
The images I upload are PNGs. I’d assume any problems are either caused by the website compressing the image(if it does that) or either handbrake which I use to extract the clip in the first place or the video editing software I use.
Handbrake is bad about compression in my past experience.