Ok I figured out what it was. If you put the parent in draft, private, or review mode, it breaks the hierarchy and WP cannot display any of its children. That seems like a bug to me.
FYI, I am creating parent pages just as groupings, not as pages that I actually want live on my site. I can hide them in other ways, but thought people should know about that one. I’ve reactivated the plugin and it seems to be fine as long as the parent is public & published.
