🚀 go-pugleaf

It doesn't widen the region before making the check.  From rmail.el:

    ;; Return t if the attributes/keywords line of msg number MSG
    ;; contains a match for the regexp LABELS.
    (defun rmail-message-labels-p (msg labels)
      (goto-char (rmail-msgbeg msg))
      (forward-char 3)
      (re-search-backward labels (prog1 (point) (end-of-line)) t))

Should be:

    (defun rmail-message-labels-p (msg labels)
      (save-restriction
	(save-excursion
	  (widen)
	  (goto-char (+ 3 (rmail-msgbeg msg)))
	  (re-search-backward labels (prog1 (point) (end-of-line)) t))))

There are several functions in the same file that do the save-restriction
via let/unwind-protect.  Not actually wrong, but not very perspicuous
either.