This is strange. When any user logs in, the admin menu and navigation menu display fine, but as soon as the user tries to navigate to another public content page, both the admin menu and navigation menu disappear. When returning to the user login page (or any admin page) Drupal indicates the user is already logged in and both menus then reappear, but only to disappear again when navigating away to a public content page.

The site never did this before and to the best of my knowledge nothing has changed for months.

Block visibility settings seem fine.