Below 0.5 bpp, both HEIF or BPG [1] and AVIF performs quite a bit better than JPEG XL , XL shines in 0.8 bpp. At least in initial testing.
There is a thread on Doom9 [1] with similar results though.
[1] https://forum.doom9.org/showpost.php?p=1894341&postcount=167