Strange lighting behaviour when generating a mesh.

Hello,

I am pretty new to unity, coming from monogame.

I recently tried to generate this mesh and light it up, but it strangely acts like there's a curve to it.

I have used mesh.RecalculateNormals(); just for you to know. Here's the video:

And code:

 Mesh mesh;
 
     Vector3[] vertices; //Vertex array
     int[] triangles; //Index array
     private float width; //Texture Width
     private float height; //Texture Height
     public int tSize = 16; //Texture size for reference
     public float splitLine = 8; //Fold Line
     private float splitNorm; //Fold line Normalized
     public Texture tex; //Texture
     Vector2[] uvs; //UV array
 
 
     void Start()
     {
         mesh = new Mesh();
 
         GetComponent<MeshFilter>().mesh = mesh;
         GetComponent<MeshRenderer>().material.mainTexture = tex;
 
         width = tex.width / tSize; 
         height = tex.height / tSize; 
         splitNorm = splitLine / tex.height;
 
         CreateShape();
         UpdateMesh();
 
     }
 
     void CreateShape()
     {
         vertices = new Vector3[]
         {
             new Vector3(0, 0, -height),
             new Vector3(0, -height * splitNorm, -height),
             new Vector3(width, -height * splitNorm, -height),
             new Vector3(width, 0, -height),
             new Vector3(0, -height, 0),
             new Vector3(width, -height, 0),
         };
 
         triangles = new int[]
         {
             0,2,1,
             0,3,2,
             2,5,1,
             1,5,4,
         };
 
         uvs = new Vector2[]
         {
             new Vector2(0, 0),
             new Vector2(0, height * splitNorm),
             new Vector2(width, height * splitNorm),
             new Vector2(width, 0),
             new Vector2(0, height),
             new Vector2(width, height),
         };
     }
 
     void UpdateMesh()
     {
         mesh.Clear();
 
         mesh.vertices = vertices;
         mesh.uv = uvs;
         mesh.triangles = triangles;
 
         mesh.RecalculateNormals();
     }