Ah, the link helps. To my mind, modular pages differ from child pages in that they are merely parts of pages and never discreet pages that can be displayed/routed on their own. Your pages are child pages because they have their own URLs, as I can see.
The underscore prefix means Grav is going to treat them as modular pages, which might have caused your recent breakage - can’t be sure.
You could simply use the “default” order now since your child pages now require a numeric prefix to be routable (
There is some overlap between modular and plain child pages, and with hacks you can probably make each of them behave like the other. I don’t feel like I have enough authority or grasp on the real intended differences to answer your question at the conceptual level. I am going to ask others to pitch in! My understanding has developed innately based on what has worked and fitted in practice
Hope that helps!