Correct, it’s the overlay header.
Again though, how does the image look to you in the post, because it’s the same URL as the one that appears in the header once you scroll down (and only once you scroll down), this is how it looks to me:
The image in the top-left and the image in the post are the same URL, it’s an SVG, scales perfectly, rendered at the quality that the browser renders it at (which should always be a pixel to pixel match with the screen resolution at the scale it’s rendered at). And it works in Firefox (linux, windows, and android), chrome (linux, windows, and android), edge (linux, windows, and android), opera (linux), and vivaldi (linux) that I tested here, they all render that SVG correctly (it’s not using any advanced effects at all, it’s as bog simple basic svg as svg can get).