unionLists.pl specifically. I think the CSS was corrected in 2.8 or 2.9. You could try the math3 theme and see if it works there. If so, then you know it is the theme issue, and you will need to update WeBWorK in order to resolve it.
What version of WeBWorK and PG are you using? In some earlier versions (I think it was around 2.7) the math4 theme included CSS that interfered with the layout of the problem. It is possible that that CSS altered the list numbering (that can be controlled by CSS), and that that is what you are seeing. I don't think it has to do with