> and was amazed that JPEG XL has more detail than AVIF at 0.05-0.1 bpp (~120KB 1080p)

bpp = bits per pixel, not bytes. At 1080p, "0.05-0.1 bpp" is 13-26 KB. Much smaller than what you were testing. Might want to recheck your sizes, if you are actually looking at 0.5 bpp it would not surprise me at all that you found JPEG XL superior.

At very small sizes like 0.1 bpp the results are debatable, but I think AVIF is a pretty clear winner if you aren't too bothered by blurring. Samples at 0.15 bpp:

https://afontenot.github.io/image-formats-comparison/#air-fo...

https://afontenot.github.io/image-formats-comparison/#citroe...

https://afontenot.github.io/image-formats-comparison/#festa-...