gp graphics - boolean operation inconsistancy

The following code
(let ((out (capi:contain (make-instance 'capi:output-pane))))
              (gp:draw-rectangle out 10 10 50 50 :filled t :foreground 
:red :operation boole-xor)
              (gp:draw-rectangle out 20 20 50 50 :filled t :foreground 
:red :operation boole-xor))
shows up differently on different systems.  

On winXP, it displays as I would expect, with the xor operations 
behaving as they should.  but on osX (10.3, lw4.3.7) the blocks that are 
display are both displayed red, seemingly unchanged by the xor 
operation.  Is this a known bug / unsupported feature of lw-osx or am I 
doing something wrong?

thanks

Andrew