Saxon

Thanks for reporting it.

I see there is an xsl:use-package declaration referencing css-xsl-package. I will need a copy of this package. It would also be good to know exactly how you are compiling and loading the package. At first sight I think it's very likely that the problem is in this area, because relatively few people are taking advantage of the package mechanism in XSLT 3.0. I imagine your CSV package is probably a variant of the one published in the XSLT 3.0 specification, but the devil will be in the detail.

Errors that aren't reproducible are often related to multi-threading. Is there any obvious multithreading in the workload here, for example executing the same stylesheet in two different threads?

Do you have a full stack trace?

It looks rather similar to

https://saxonica.plan.io/issues/5148

which was fixed in the 10.7 maintenance release. Is there any reason you haven't moved forward to that release?