suddenly, mesh modified by perlin noise is symmetric

so i am a very beginner when it comes to coding, and i copied this script from a youtube tutorial to generate infinite terrain.
actually it’s two scripts. one is modifying the mesh of a prefab plane, the other is instantiating these planes according to the players position.

so i had the idea to check if the vertices wich are getting modified are greater or less than zero, so that i could instantiate another prefab with actual premade terrain, that doesn’t get affected by the code.

i made an if statement "if (vertices[v].y and so on… else do nothing. after some fiddlinge it worked…
but suddenly all the terrain was symmetrical on the z axis from the players starting position…

i removed the code, but it remained that way, even when i loaded a backup of that project.

maybe it was that way since i copied that script and i just didn’t noticed, but i am very sure that isn’t the case.

someone can help me out?
sorry if my english is not perfect, i am not a native speaker.

here’s the code that modifies the mesh: void Start()
Mesh mesh = this.GetComponent().mesh;
Vector3 vertices = mesh.vertices;

    for (int v = 0; v < vertices.Length; v++)
        vertices[v].y = Mathf.PerlinNoise((vertices[v].x + this.transform.position.x ) / detailScale,
                                          (vertices[v].z + this.transform.position.z ) / detailScale) * heightScale;

nevermind, i guess i figured it out. apparently i just didn’t notice it before…