[plt-scheme] paren-shape bug?

I am trying to debug a paren-shape property loss bug, and I have arrived 
at this little case:

proptest.ss:
#lang scheme/base

(require "mzprop.ss" (for-syntax scheme/base))
(provide (all-defined-out))

(define-syntax (propt stx)
   (syntax-case stx ()
     ((_ hd)
      (with-syntax ((shape (syntax-property #'hd 'paren-shape)))
        #'(quote shape)))))


mzprop.ss:
#lang scheme/base
(provide (all-defined-out))
(define-syntax propbotch
   (syntax-rules ()
     ((_ K in ...)
      (K [in ...]))))


$ mzscheme
Welcome to MzScheme v4.1.5.4 [3m], Copyright (c) 2004-2009 PLT Scheme Inc.
> (require "mzprop.ss" "proptest.ss")
> (propt [a b c])
#\[
> (propbotch propt a b c)
#f

What happened to my paren-shape?

-- vyzo