emacs -Q

In *scratch*, evaluate:

(funcall
 (let ((lexical-binding t))
   (byte-compile
    (lambda (&optional a b)
      (list a b))))
 'a 'b)

Result is (a b), as expected.  Now evaluate

(funcall
 (let ((lexical-binding t))
   (byte-compile
    (lambda (&optional a &optional b)
      (list a b))))
 'a 'b)

Result is (b nil).

Emacs should either generate correct code in both cases, or signal an
error in the latter case.