The result of the inner crossed-dyadic product of two symmetric tensors $\boldsymbol{A}$ and $\boldsymbol{B}$

$$C_{ijkl} = A_{ik} \cdot B{jl} $$

is unsymmetric and hence, can't be stored as a 6x6 tensor.

$$C_{ijkl} \ne C_{jikl} \qquad \text{and} \qquad C_{ijkl} \ne C_{ijlk} $$

The implementation is wrong.