Signs are lighter/darker due to their rotation

Standings Signs in the rotations 11, 12, 13, 14, 3, 4, 5 and 6 have the texture of the sign, where the text appears, darker.
This effect occurs from any angle.
NOTE: It seems that the displayed text is lighter, which is NOT the case. I analyzed the colors and they are exactly the same.

Testing was done in a DEFAULT world generated with the related snapshot.

For a more detailed look, watch: https://youtu.be/k3b7onVxvQQ