Ah, thanks a lot! I would have never guessed this... Maybe the documentation should say something like: "Querying across elements is only supported when whitespace chopping is off." If it's ok with you, I'll add it.
..thanks again for editing our Wiki -- always welcome! I've slightly added your last paragraph to indicate that full-text tokenization always works on the string values of the elements:
http://docs.basex.org/wiki/Full-Text#Mixed_Content
Feel free to further polish it, Christian