All buttons in a button set becoming selected

I am using Storyline 1 and I have several multiple-choice questions on a single slide. The answers for each question are in their own separate button set. However, in preview and in the LMS, choosing either response causes both to be selected (pretty much the opposite of what a button set is supposed to do). You can see it in the attached screenshots. Even hovering over one of the radio buttons causes them both to be highlighted.

There are no variables or triggers associated with these buttons - the questions are just for self-checking so they are not graded.

Any ideas what is causing this and how to fix it? Thanks!