Question Rigidbody with X and Z frozen, stops falling after colliding with a static body

I seriously couldn't find if this problem was asked somewhere, so I will try to ask here, as my first post on this forum, about this problem that is happening.
I have a 2d "sand" block which can fall, but can't move X direction. Also, I have a static block, which can't move in any direction. If I place two sand blocks like the image, they manage to fall.





But if I place it next to a static block, it stops falling, like the picture.



Here is the piece of code I use to pace the blocks, where I try to "simulate" a 2D grid:

Code (CSharp):

1. void PlaceBlock(){
2.         Vector3 ct = GameObject.Find("Cursor").transform.position;
3.         block.transform.position = new Vector3(Mathf.Round(ct.x), Mathf.Round(ct.y), (int)ct.z);
4.         Instantiate(block);
5.         canPlaceBlock = false;
6.     }

Thank you in advance.