Marking as confirmed, and changing the severity to Minor.

The first two instances, where Octave returns a string but Matlab returns a char matrix, are the ones to focus on.

The code to start looking through is in graphics.in.h and the class definition for string_array_property.  The class provides an overload for the string_value() function that creates the pipe-delimited list.

  std::string string_value (void) const
  {
    std::string s;

    for (octave_idx_type i = 0; i < str.numel (); i++)
      {
        s += str[i];
        if (i != str.numel () - 1)
          s += separator;
      }

    return s;
  }

There would probably need to be a new conversion function with an output of char matrix rather than std::string.  This conversion function would essentially just call Fchar() on the list of strings.  And then there would be changing the get function for "string" so that it returned a char matrix so it would pick up the new overload.