Admin and Page Template?

The modular.html.twig page template from my theme is not showing up in the Admin as a choice for new pages types. My other page templates are. What is it that tells the Admin to include, or not include, a page template?

The Admin plugin scans the templates/ folder of your theme, you can even put an empty file called modular.html.twig and "Modular" will show up in the list.
If you set Antimatter as your theme, does it show up?

OK, I got this fixed. But I'm not sure how. It may have been a sticky cache - I know a couple of bin/grav clear-cache --all happened along the way.