overloading subsref() in a Heterogeneous class

I would like to overload subsref() in my class, however I noticed that whenever I use Heterogeneous arrays in my class definition, MATLAB does not allow me to overwrite subsref() :

classdef testsubsref < matlab.mixin.Heterogeneous & handle % produces an error for subsref()
% classdef testsubsref < handle                               % does not prodcue an error for subsref()

      properties
          x=0;
      end

      methods 
          function obj = testsubsref(val)
              if nargin>0
                  obj.x=val;
              end
          end

          function y = f(obj)
              y=2*obj.x;
          end

          function sref = subsref(obj,S)
              fprintf('in subsref \n');
              sref = builtin('subsref',obj,S);
          end
      end

end

For example,

c=testsubsref(1)
Error using testsubsref
Class 'testsubsref' is not allowed to define a 'subsref' method.

When matlab.mixin.Heterogeneous is removed from the class definition the subsref() works as expected.

My guess is that matlab.mixin.Heterogeneous uses subsref() for its imeplementation, and makes it Sealed (?)

Any suggestion how to make a subsref() for a class with heterogeneous arrays?

Thanks Arnon