e.g., specifying the :DEFAULT-INITARGS option to DEFCLASS (with a null list of default initargs in this case) changes the class's direct default initargs, not specifying it has no effect when the class object is reinitialized. This is consistent with the MOP's model of class redefinition (where it ultimately involves REINITIALIZE-INSTANCE on the class object using a well-defined set of initargs derived from the expansion of DEFCLASS.
Implementations do differ in their treatment of your example: the versions of LispWorks? and CLISP that I have access to behave as you expect and the versions of SBCL, Allegro, and CCL behave the way that I think is correct. I don't think that CLHS says anything about what happens in this case, but (as I said) I think that what the MOP says is quite clear.