Hi Patrick,
I am thinking there is some sort of bug in Storyline regarding the freeform drag and drops.
Another user this morning had the same issue: https://community.articulate.com/discussions/articulate-storyline/skipped-slide, and it seems like some freeform drag and drop slides are automatically triggered to jump to the next slide.
I was able to correct it by creating a new slide, moving all of the objects, layers and triggers from your non-working slide into the new one, and converting it to freeform, but I hope Articulate staff will take notice. In the meantime, you might want to submit this as a support case: https://articulate.com/support/contact/360-teams