Modular predictors (`j40__modular_channel` and related functions in J40). The JPEG XL specification was written after the reference implementation, and there are lots of bugs especially in that part of specification. There are also a significant number of bugs in DCT-like transformations, but those bugs are relatively isolated while modular predictors can wreck the entire image. Those bugs are being reported to the spec editors, so later implementations will have a much easier time.

I really appreciate the work that must have gone into narrowing down those bugs. There's a reason for the insistence on multiple independent implementations for new standards; the Internet owes you one.