Views file name suggestions often missing from Twig debugging

(Drupal 8.2.4, using a custom theme that has 'stable' as the base.)

I'm not sure if I've broken something, but (having rebuilt the cache etc.) the FILE NAME SUGGESTIONS section is missing entirely from the Twig debugging for view pages:

<!-- THEME DEBUG --><!-- THEME HOOK: 'views_view' --><!-- BEGIN OUTPUT from 'core/themes/stable/templates/views/views-view.html.twig' -->

… when I copy the template from stable to my theme and give it a more specific filename (for a view with a machine name of "gifts"), I get this:

<!-- FILE NAME SUGGESTIONS:
   x views-view--gifts.html.twig
   x views-view--gifts.html.twig
   * views-view.html.twig
-->

which:

a) doesn't list any of the other options (i.e. ID, display type)
b) has the same entry repeated twice

Any ideas?