Is the groove on the back of the tile deep or shallow?

The groove is deep. It's good. The groove on the back of the tile is deeper, which helps to make the tile stick more closely. Even if the tile and cement mortar layer fall off locally, it will not cause the bricks of the whole wall to fall off. If the groove is too shallow, it may affect its mechanical properties and water resistance, and may even lead to cracks on the surface of the tile in the future.