Same result, the featured image in the first post gets pixelated.
Oddly, when I set in Customizer the height for the featured images, it doesn’t work until I type Auto in the width box.
I mean, Auto is set by default and I can see it in grey colour. Then, I set e.g. 200px in height box, but nothing changes neither in preview nor saving changes.
I have to manually type Auto in the width box (it gets black colour) to make the featured images resized to the height I want.
I have tried deactivating the lightbox plugin with same result.
(I don’t know if this has something to do with the goal I want for the first featured image)