Now that you say it, I can see a clear downgrade from the screenshot #5 to the #6, but only if I open the images on a new tab at a larger scale.
Of course you have to open the images on a new tab!
Otherwise you don't compare 4K to 1440p to 1080p to 720p images, you are comparing 546p-images downscaled from different resolutions!
This forum is fixed for 1200 pixels horizontal (including avatar pix and frames) and the area left for pictures is 972 pixels horizontal. That leads to downscaled 16:9 screenshots in 972 x 546, so even lower than 720p: