The auto generated index discards entries in the
"symbols" index group based on which character they
start with. I've attached a mimimal docbook document
demonstating the problem. I've noticed the problem with
the docbook-xsl versions 1.51.1, 1.50.0, and 1.49
release for both FO and xhtml output.
I've found another similar but different problem with
indexterms while tracking this one down that I will
describe in my next bug report, you may want to address
it at the same as this one as they probably related to
the same block of code. I would include a patch but I'm
not quite sure how to address this problem cleanly.