In a recent Emacs master (as provided in Silex's Docker image):

$ emacs --batch --eval "(with-temp-buffer (insert
\"\n;;;###autoload\n(defun ...)\n\") (goto-char 1) (print
(re-search-forward \"^[[:space:]]*;;;###autoload[[:space:]]*$\" nil t)))"

Prints nil.

With every stable version of Emacs I have this prints 16.

Paul