I'm going to try all this some night when it's very late and I can log in as the student (she gave me her password) without worrying that she might want to be logged on to do homework.
The screen shots I posted are all from my laptop, using the same browser (Firefox). I think you have hit on the processing that is leading to the student-screenshot (and leaving the ? in place), but I'm starting to think it has to be something about the student's account in the database. All my testing was done from the same computer and browser. The problem is consistent when logged in as this student (and it happens for the surrounding problems which are the same structure, using a variety of seeds) and is consistently not a problem for other users, even with the same seed. It's this consistency that makes me tend to agree that this is not about randomization. (Maybe there is something about her name itself that is causing trouble with the scrubber?)
We are on a develop branch of pg that was pulled in mid September, so I think that parserPopup.pl is up to date. But I'm curious to see what would happen if I recoded the problem to not mess with correct_ans_latex_string. I'm going to try many things, but it's awkward to find enough time to devote to this given that I will have to cut off the student by changing passwords and raise her permission level to professor to experiment with changes. And I'm going to be content once I see the issue go away, even though I'd ideally like to fully understand what leads to it.