I'm experiencing a frustrating sorting issue with a Tilemap in Unity (Isometric layout). I have three block types: Grass, Sand, and Water.

Sprite Assets Specifications:

• Dimensions: All sprites are exactly 500x500 pixels.

• Pixels Per Unit (PPU): All are set to 512.

• Pivot: All use an identical custom pivot of (250, 375) (which corresponds to the bottom center of the top diamond of the cube).

• Geometry: The sprites are identical in shape and metadata; only the colors and filenames differ.

The Issue:

1. If I assign the same sprite asset (e.g., just the Grass sprite) to all three slots in my generator, the chunk renders perfectly. The $Y$-axis sorting works exactly as expected.

2. As soon as I use three different files (Grass.png, Sand.png, Water.png), the sorting breaks completely. Background tiles frequently render on top of foreground tiles, creating a "layer soup" effect.

using UnityEngine;
using UnityEngine.Tilemaps;
using UnityEngine.Rendering;
using System.Collections.Generic;

public class WorldGenerator : MonoBehaviour
{
    [Header("Sprites")]
    public Sprite waterSprite;
    public Sprite sandSprite;
    public Sprite grassSprite;

    [Header("Settings")]
    public int chunkSize = 16;
    
    private Dictionary tileCache = new Dictionary();

    void Start()
    {
        // Setting up global sorting for isometry
        GraphicsSettings.transparencySortMode = TransparencySortMode.CustomAxis;
        GraphicsSettings.transparencySortAxis = new Vector3(0, 1, -1);

        GenerateSingleChunk();
    }

    void GenerateSingleChunk()
    {
        GameObject oldGrid = GameObject.Find("IsoGrid");
        if (oldGrid) DestroyImmediate(oldGrid);

        GameObject gridObj = new GameObject("IsoGrid");
        Grid grid = gridObj.AddComponent();
        grid.cellLayout = GridLayout.CellLayout.Isometric;
        
        // Size for 500px sprites with 512 PPU
        float sw = 500f / 512f;
        grid.cellSize = new Vector3(sw, sw * 0.5f, 1f);

        GameObject chunkObj = new GameObject("Chunk_Main");
        chunkObj.transform.SetParent(gridObj.transform);
        
        Tilemap tilemap = chunkObj.AddComponent();
        TilemapRenderer tr = chunkObj.AddComponent();

        tilemap.tileAnchor = Vector3.zero; 
        tr.mode = TilemapRenderer.Mode.Individual; 
        tr.sortOrder = TilemapRenderer.SortOrder.TopRight;

        for (int x = 0; x < chunkSize; x++)
        {
            for (int y = 0; y < chunkSize; y++)
            {
                tilemap.SetTile(new Vector3Int(x, y, 0), GetRandomTile());
            }
        }
    }

    Tile GetRandomTile()
    {
        float r = Random.value;
        if (r < 0.33f) return GetCachedTile("Water", waterSprite);
        if (r < 0.66f) return GetCachedTile("Sand", sandSprite);
        return GetCachedTile("Grass", grassSprite);
    }

    Tile GetCachedTile(string id, Sprite s)
    {
        if (!tileCache.ContainsKey(id))
        {
            Tile t = ScriptableObject.CreateInstance();
            t.sprite = s;
            tileCache[id] = t;
        }
        return tileCache[id];
    }
}

screenshot#1

screenshot#2

What are the possible ways to resolve this sorting conflict?