Next button trigger keeps changing

Well, never mind. Of course, as soon as I post the question, I see the answer, so I'm posting it here in case anyone runs into it.

If I change the next button trigger to something other than advancing to the next slide, and I have not yet set the slide to advance automatically, the view of the scene in the scene panel or in story view shows that the slide is no longer attached to the one after it, because there is at the moment no path from the slide to the next slide. If you then drag that next slide, under the current slide, THAT action changes the next button trigger behind the scenes.

So, the solution is to change whether the slide advances automatically or by user BEFORE you edit that trigger. Or else don't be tempted to drag your slides around in story view because they look wrong temporarily.