Mesh.CombineMesh() results in different heights between objects

Hi, I'm trying to create a Minecraft like Terrain. So each Chunk contains 16 * 16 blocks which should be combined into one mesh. To reach this, I wanted to use Mesh.CombineMesh(); The problem is, that the height between the Chunks is different after I created the Chunk mesh.

I won't have this problem if I don't combine the mesh. Is there a way to remove the height difference between the meshes?

This is my code, it might help! Thankful for any help!

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Chunk : MonoBehaviour {
 
     public void generateChunk(int chunkWidth, int chunkHeight, int chunkLength, float heightModifier, float perlinSize){
         List<MeshFilter> meshfilters = new List<MeshFilter>();
         for(int x = 0;x < chunkWidth;x++){
             for(int z = 0;z < chunkLength;z++){
                 float h = Mathf.PerlinNoise((gameObject.transform.position.x * perlinSize) + (float)(x * perlinSize), (gameObject.transform.position.z * perlinSize) + (float)(z * perlinSize));
                 h *= heightModifier;
                 GameObject block = GameObject.CreatePrimitive(PrimitiveType.Cube);
                 block.transform.SetParent(gameObject.transform);
                 block.transform.localPosition = new Vector3(Mathf.RoundToInt(x), Mathf.RoundToInt(h), Mathf.RoundToInt(z));
                 meshfilters.Add(block.GetComponent<MeshFilter>());
             }
         }
 
         CombineInstance[] instance = new CombineInstance[meshfilters.Count];
         for(int i = 0;i < meshfilters.Count;i++){
             instance[i].mesh = meshfilters[i].sharedMesh;
             instance[i].transform = meshfilters[i].gameObject.transform.localToWorldMatrix;
             meshfilters[i].gameObject.SetActive(false);
         }
         gameObject.AddComponent<MeshFilter>().mesh = new Mesh();
         gameObject.GetComponent<MeshFilter>().mesh.CombineMeshes(instance);
         gameObject.GetComponent<MeshFilter>().mesh.Optimize();
         gameObject.GetComponent<MeshFilter>().mesh.RecalculateBounds();
         gameObject.AddComponent<MeshRenderer>().material = meshfilters[0].gameObject.GetComponent<Renderer>().material;
     }
 }