Code Completion does not work for operator-() result with internal code model

Some code excerpt of the attached project:

// operator- returns a DoubleDiff
DoubleDiff Double::operator-(const Double& sub) const
   {
   return DoubleDiff(_double - sub._double);
   }

// DoubleDiff has a method double diff() const;

Double d1(100.);
Double d2(50.);

double diff = (d1-d2).diff();

If you place the cursor on diff() it is not detected as method of DoubleDiff.

If you write (d1-d2). and then request Ctrl+Space code completion you will get the mtehods of Double, but not of DoubleDiff.

Comment: With clangd code model this works.