Is there something wrong with the AVIF decoder's chroma upscaling? E.g. look at "Kodim 3", where the colored hats overlap each other. There's severe blocking, where I'd expect blurring instead. All the other formats blur, which is much less distracting.
I've noticed that too. I've been testing AVIF, and the reference encoder[0] defaults to 4:4:4 (no chroma downsampling) if you don't tell it otherwise. I tried 4:2:0 and it looked horrible, but the size hit for 4:4:4 wasn't bad and looked much better.