Weird problem with doc()

Using XSLT 2.0, I'm using doc() to access data in an outside XML file. This is
running inside the DITA Open Toolkit, which is an Apache Ant application.
doc() accesses the whole file and displays the text content of elements when I
do this:

OUTSIDE FILE -

This is expected. But when I add some XPath to get to certain data inside the
file like below, the output goes blank:

OUTSIDE FILE -

The XPath is correct because I can run this exact code outside of the DITA
Open Toolkit and it works as expected. I realize there is a lot going on
inside the DITA OT but I'm just curious what kind of thing could cause this
strange behavior of doc() being able to access the file, but failing with
XPath. There is no error reported.

It's possible that the DITA OT code my XSLT is running inside of (Oxygen
Webhelp) is calling XSLT (Saxon processor) from Java, and my wild guess is
that I am running into some kind of JVM sandbox that won't let random code
pull in outside stuff. Does that make any sense?

Thanks,
Mark