In most of the cases I found cubic structures are considered to be isotropic.

Please check your theoretical calculations to confirm.

Also check your assumptions and constraints.

It will be contradictory to the experimental results.

Single crystals are all anisotropic which is already clear by the strong orientation of bondings between atoms. Also cubic crystals with high symmetry are anisotropic, but it depends not only on the crystal itself but mainly on the property you are discussing. These are described by tensors. Thus the mass density is isotropic for any material whereas double refraction does not exist for cubic materials.  There are very nice presentations but also book in internet available which give you a more reliable answer than a question here at ResearchGate. People simply do not answer if questions becoming too fundamental and entire books are required to explain this topic or ton of material is available in internet. Please use open sources before you start to ask a question and make the conclusion that if nobody reacts nobody knows the answer.

I attached a link which was one of the first hits if you enter isotropy and anisotropy in crystals.

http://www.crystallography.fr/mathcryst/pdf/AsCA-MaThCryst-Tsukuba-06_Ferraris.pdf

Everything depends on the composition. Doped perovskites must be anisotropic even if the structure is cubic.

A property is a relation between two measurable quantities / variables namely independent and dependent and it is a tensor.  The number of non-vanishishing independent  components of that  tensor property varies from structure to structure or depends on symmetry of the the crystal.  Therefore, for a given crystal system depending upon its  symmetry the number of non-reducible independent components depends.  Hence, a crystal not only isotropic or anisotropic  but if it is anisotropic no. of components also depends on symmetry. For details

one can see the book "Physical Properties of Crystals-Their Representation by Tensors and Matrices by J. F. Nye.

As mentioned and explained by colleges above, crystals are anisotropic.  This occurs also by others materials like polymers because molecular orientations, with or without fillers (fibers for example).  Symmetry and spatial organization of atoms and molecules are the reason.  During processing of polymer (molten and later cooled until solid state) their molecules get oriented mainly in the flow direction.  Also deformation in solid or plastic stage like stretching, thermoforming produces orientation in molecules and the material becomes anisotropic.  Polymer without molecular orientations or with randomly distributed fillers, in case they are present, are or can be treated as isotropic.

some materials prepared by powers maybe  isotropic because of the isotropic powers, but when they repreduced by the seconed processes, just like die-upset, squeezing, rolling, the isotropic power may be combine or grow up to anisotropic single crystals .

I completely agree with answers of Gart Nolze, G. Bhikshamaiah and Alberto Naranjo. The isotropic or anisotropic property means orientational property in space. Even for a simple cubic system with same type of atoms sitting on

all corners, only three perpendicular directions are isotropic. The body diagonal

and face diagonal are not isotropic to each other.