Re-exported typed signature recognized as untyped signature

I'm not sure if this is a bug or just an intended behavior because there's no contract protecting signatures flowing from untyped to typed.

What version of Racket are you using?

v6.9.0.2

What program did you run?

File untyped-sig.rkt:

#lang racket
(provide sig^)
(define-signature sig^ (n))

File sig.rkt:

#lang typed/racket
(require/typed "untyped-sig.rkt"
  [#:signature sig^
   ([n : Number])])
(provide sig^)

File main.rkt:

#lang typed/racket

(require "sig.rkt")

(define-unit u@
  (import)
  (export sig^)
  (define n 42))

What should have happened?

Program ok

If you got an error message, please include it here.

; Error in macro expansion -- use of untyped signature in typed code;
;  consider using `require/typed' to import it
;   signature: sig^
;   in: (define-unit u@ (import) (export sig^) (define n 42))

If I copy the require/typed clause over, the program works as expected:

#lang typed/racket

(require/typed "sig.rkt"
  [#:signature sig^
   ([n : Number])])

(define-unit u@
  (import)
  (export sig^)
  (define n 42))

(define-values/invoke-unit/infer u@)
n ; ==> 42

Start by running main.rkt with the provided sig.rkt and untyped-sig.rkt files, then compare it with the working require/typed example. Trace how the re-exported sig^ is classified during define-unit macro expansion; done means the first program runs successfully without copying the require/typed clause.