I have a parent theme with templates defined in a directory called templates. Within that folder, I have directories like base (page, html, block etc twig templates), views, and menu folder.
My subtheme, which is based on the parent, will not load these templates even though base_theme is set in the .info.yml file.
What am I missing? If I copy the templates into the subtheme, they begin to work.
Annnnd after asking I read my question and realized my answer. The key base_theme is not one word, it's two.
Make sure you have the correct indentation in THEME.info.yml file, I had some problems before with this aspect. And also clear the cache after changes.