[Bug: 21.4.19] customize-face loses display specs information

[Stephen J. Turnbull]

ignotus at freemail.hu writes:

 > When I set multiple specs using M-x customize-face then when I customize
 > it again later it looses the non-current spec specific information.  I
 > know it is hard to understand, here is a short example:
 > 
 > (setq custom-face-default-form 'selected) ; this is the default
 > 
 > 1) M-x customize-face RET default RET
 > 
 > 2) go to [state] button, RET 7 (show all specs)
 > 
 > 3) set the spec of your current session, then add a new spec

What spec do you add?

 > 4) save it, kill the customize buffer
 > 
 > 5) M-x customize-face RET default RET

You haven't changed anything, but saving does something?

 > 6) save it, now you have lost the extra spec you added before