what MAME version were you using as your XML input?
There was something in the 135 and higher releases that was tripping up the list outputs, which I thought I fixed with v22, but it may be possible that something was missed.
pop me an email jeff at silent dot net, and we will get to the bottom of this. you may need to zip up your xml file and post it somewhere i can get to it to try and reproduce the issue.
another thing would be to output the list rom names only or something else in text format, and then take a look. if the names turn to mush halfway down the list, then i have a bug to fix.