⚡ Quick Fix (TL;DR)

The Culprit: debug.log, the server-level error log, WPCode's own fatal-error auto-deactivation, a custom register_shutdown_function fatal catcher, and a custom set_error_handler catching every warning and notice all came back completely empty, despite the bug being fully and repeatedly reproducible. Whatever was happening wasn't a standard PHP fatal, warning, or notice that any of these layers are built to see.

The Fix: Bypass every error-reporting layer entirely and trace execution by hand: drop plain, unconditional markers directly into the code with file_put_contents(), writing to a plain text file outside any logging system. Compare which markers do and don't appear across test runs to pinpoint exactly how far execution gets before something goes wrong – this was the only method all session that produced any real signal.

function zfn_debug_mark( $label ) {
  file_put_contents(
    WP_CONTENT_DIR . '/uploads/your-debug.log',
    date('c') . " - $labeln",
    FILE_APPEND
  );
}
zfn_debug_mark( 'reached this point' );