It is possible you have discovered a bug. I don’t know too much about what is going on, but rendering your original file with a current svn build I get this:-
Try looking on graphicall.org for a recent build for your system and render the file with that. If it looks the way you wanted it to, it could be worth filing a bug report.
Go to https://projects.blender.org/
I think you need to set up an account, then check the bug tracker to see if it is already reported, and if not report it.
Good luck, and well done if it is one.
EDIT - It looks like someone reported a similar bug back in March (for 2.46 - I guess it was fixed since) so I would say go ahead and report it - mention that it seems to have been fixed in svn and see how it goes.
Here is the older report:-
Chances are the developers are aware of it, but it can’t hurt to mention it again. If it was fixed for 2.47 it may have broken again in 2.48.