Collider doesn't work. Adding a second identical collider in-game fixes the problem.

I'm making an endless runner where I spawn the tiles as the player moves. I've found that for some random tiles, the player just sinks down the tile, ignoring the BoxCollider. However, if I add a second BoxCollider in-game, everyt$$anonymous$$ng works fine. How do I fix t$$anonymous$$s problem?

Here is a link to a gif demonstrating the problem:

https://gfycat.com/ConcernedAbsoluteBeaver

T$$anonymous$$s is the snippet to move the character forward. The first "if" was added for debugging purposes to ensure that the player doesn't fall-through infinitely

 void Update () {
     Debug.Log("controller.isGrounded : " + controller.isGrounded);
     if (transform.position.y < 0.5) {
         speed = 1.0f;
         return ;
     }
     Vector3 moveVector = Vector3.forward * speed * Time.deltaTime;

     if (Input.GetKeyDown("a")) {
         moveVector.x = moveVector.x - tileBreadth / 3;
     } else if (Input.GetKeyDown("d")) {
         moveVector.x = moveVector.x + tileBreadth / 3;
     }

     if (!controller.isGrounded) {
         moveVector.y = moveVector.y - gravity * Time.deltaTime;
     }
     controller.Move(moveVector);
 }