The Z-axis side of Spawner has a visual difference with other sides

When I put solid block right next to spawner, the side of the spawner that touches the block disappears, while the opposite side remains. but when I put the block on north(or south) side, it works the other way around - remove the south(north) side and keep north(south) side.

 

( +adding screenshots, each one includes sun or sunflower to notice the directions)