The srfi text says 'getter' argument of register-getter-with-setter! takes two arguments, the object and the field identifier.

The reference implementation, however, may call 'getter' with three arguments, if the object is sparse or 'ref' is called with the default argument.

Typically srfi text is definitive, but in this case, limiting getter to two arguments makes it impossible to implement three-argument 'ref'.  So I suggest this should be post-finalization correction.