Wrong slides get lightboxed intermittently

I'm trying to create a simple interaction for a medical tray image. Learners hover over each instrument on the tray, click on a "?" icon that appears, and complete a lightboxed freeform or graded slide interaction about that particular instrument. Upon successful completion of each interaction, the lightbox closes and a checkmark disables that instrument's hover/click capability. Sounds simple, right?

Trouble is, my lightbox triggers *intermittently* lightbox the wrong slide (usually it picks the lowest numbered slide/scene), so there is a mismatch between the instrument and the question. (Sometimes it also loads a blank slide instead.)  It's not consistent but almost always affects at least one instrument trigger, making the interaction glitchy and unworkable for users. I have tried deleting and recreating the trigger buttons, deleting and recreating some of the interaction slides, all to no avail. 

Anyone recognize this issue? Is there some underlying lightbox setting where it defaults to the next slide/scene rather than the triggered slide? Could there be a performance issue here, even with such a small course? Grateful for any insights y'all might have.

Thanks and file attached!   - Ian

P.S. For recreating, the most common issue seems to be with the 'next' slide (2.1 speculum knowledge check) appearing in place of the appropriate slide (e.g. 3.1 aspirator knowledge check, when that ? icon is clicked. But it might take a couple rounds of clicks for it to show up.