The piston_side texture is not rotated properly along the bottom of the piston body when facing along the z axis.
The attached photo shows the improperly rotated texture along the z axis, and the properly rotated texture along the x axis.

Steps to reproduce:

1.) Download the texture pack given.

piston_texture [MConverter.eu].mcaddon
2.) Apply it to any world.
3.) Place a piston/sticky piston so that the head is facing along the z axis.

4.) Check the underside of the piston.

Observed: The texture does not rotate consistently on all sides.

Expected: The texture should be rotated properly on all sides.