If there exists a conserved energy density then the modulus tensor Cijkl is equal to Cklij. The corresponding compliance elements have the same symmetry. If there does not exist a conserved energy density, these moduli or compliance values can be unequal. That is entirely distinct from the asymmetry of the stress tensor that occurs in Cosserat elasticity. Asymmetric physical property tensors have recently been called odd.
Asymmetry in the compliance tensor has been thoroughly observed experimentally in wood which is orthotropic and viscoelastic and has long been known. All of the 12 compliance components have been determined experimentally vs. time for various woods. The different time dependence of the non-diagonal elements of the compliance matrix reveals the odd nature of the viscoelastic character of wood though the word odd was not used at the time. The theoretical basis has also long been known.
References
H. Neuhaus, Elastic behavior of spruce wood as a function of moisture content, Holz Roh-Werkst. 41 (1), 21-25 (1983).
Ozyhar, T., S. Hering, and P. Niemz, 2013, Viscoelastic characterization of wood: Time dependence of the orthotropic compliance in tension and compression, Journal of Rheology 57(2), 699-717 {2013}.
S. Hering, D. Keunecke, and P. Niemz, Moisture-dependent orthotropic elasticity of beech wood, Wood Sci. Technol. 46, 927-938 (2012).
A. C. Pipkin and G. Rogers, Asymmetric relaxation and compliance matrices in linear viscoelasticity, ZAMP, 14, 334-343 {1963}.