The problem I have is that after a student takes one of the quizzes, that quiz's results then get hidden in the gradebook.

It seems to be specific to just the first of the quizzes (there are three), but it happens with any student.

I can't see what's different between the quizzes that would make it behave this way.

I've attached the sample course that I'm using to test this. You'll see that the quizzes each take random questions from two of four categories. I use this course to demonstration the random question capability.

I'm using Moodle 1.9+ build 20080423.

Forum thread: http://moodle.org/mod/forum/discuss.php?d=101573