Playing with mpv.
They appear mostly on curvatures when the screen is "moving" vertically.
Yeah :D if you're familiar with the story, then it's Sakura's and Ino's duel.
While the aspect ratio is such that the picture is a square with some empty space on the sides, I don't understand how a measly 720x480 can look this "unpixelated" on my 43 inch 4K TV screen...