Bug was: in Eigen matrix functions, passing a matrix expression by reference means
that each time the reference is needed the expression was reevaluated.
This produced that when calling a function
  e.g. f( Matrix::Random() ),
the parameter passed is an expression and not a matrix. Therefore, each
time the matrix is used in f(), a new Random was created.

Solution: use the .eval() method to evaluate the expression:
as in
  e.g. f( Matrix::Random().eval() )