Saxon

Yes, I'm confused too. This is one of the older Saxon extension functions and it is implemented as a reflexive function returning a Java byte[] array. There's a bit of a history of problems with Java->XPath conversion of byte arrays. It looks at first glance as if the run-time semantics is unsigned byte, but the static type inferencing is assuming signed byte; the solution is probably to change the static semantics to match the dynamic.

This will be followed up here:

https://saxonica.plan.io/issues/3525