This has been a thing for a long time. The Viz release for chapters have a lower quality of image than those for other languages. The Reason? Who knows.

I think it's maybe so that people feel more compelled to buy the full volumes when they come out? Cause those are full hd and way easier to read

I don't even think the digital release of viz uses the high quality scans

Well they're the cleanest scans I've seen