⚡ Quick Fix (TL;DR)

The Culprit: register_post_type() is commonly called on init, but WordPress builds the actual admin sidebar menu on a separate, later hook called admin_menu. A clean marker on init only proves the post type object itself registered – it says nothing about whether the later menu-building step actually succeeded. Trusting an init-only marker as proof the menu would appear cost a full round of false-negative testing before this was caught.

The Fix: When testing whether something admin-menu-related is actually working, add a diagnostic marker directly on admin_menu itself (a late priority like 999 ensures it runs after everything else on that hook) – test the specific hook that governs the real behavior in question, not just the nearest hook that happens to be convenient to log.

add_action( 'admin_menu', function() {
  // confirms the menu-building step itself actually ran
  zfn_debug_mark( 'admin_menu fired' );
}, 999 );