Well, for the buttons I'm using (for touch input), mesh colliders don't show a z-axis option. Is Trigger/Convex/Smooth Sphere Collisions are all unchecked, no material and mesh is the default.
For the bullet, there's a box collider, and the zsize is not touching the camera, and is wide enough to collide with any/all objects (it hits enemies properly). I've added a rigidbody/kinematic to the bullet, but it does not change the reaction. (going step by step, the collider on the bullet does cross the collider on the ground).
1. Yes, the sprites/colliders are moving together. I do not have a rigidbody on the sprite, as it didn't change anything. (I do now, no change). (Adding one on the ground sprite that the bullet first hits does nothing as well)
2. Box colliders on both.
3. Yes, they are intersecting.
4. The bullet is moved with it's own script: transform.position.x += (1000 * Time.deltaTime)