Strange artifacts when porting GLSL to Metal

I tried to port this shadertoy:

https://www.shadertoy.com/view/4dX3zl

to Metal with as little changes as possible.

But there are little artefacts when I run it on Mac or iPhone: some voxels would sometime "jump" out of place. This happens with both - branchless and ordinary versions.

The original shadertoy runs nicely in a browser without any issues.

I wonder what causes these artefacts and how can I get rid of them...

Here is the Swift+Metal code that you may simply run in a Playground or in Swift Playgrounds app:

import MetalKit
import PlaygroundSupport

public let metalFunctions = """
#include <metal_stdlib>
using namespace metal;

        constant constexpr int MAX_RAY_STEPS = 64;

        float sdSphere(float3 p, float d) {
            return length(p) - d;
        }
        float sdBox(float3 p, float3 b) {
            float3 d = abs(p) - b;
            return min(max(d.x, max(d.y, d.z)), 0.0) + length(max(d, 0.0));
        }
        bool getVoxel(int3 c) {
            float3 p = float3(c) + float3(0.5);
            float d = min(max(-sdSphere(p, 7.5), sdBox(p, float3(6.0))), -sdSphere(p, 25.0));
            return d < 0.0;
        }
        float2 rotate2d(float2 v, float a) {
            float sinA = sin(a);
            float cosA = cos(a);
            return float2(v.x * cosA - v.y * sinA, v.y * cosA + v.x * sinA);
        }
kernel void shader(texture2d<float, access::write> out [[texture(0)]], constant float& time [[buffer(1)]], constant uint2& viewportSize [[buffer(0)]], uint2 tid [[thread_position_in_grid]]){
    if (tid.x >= viewportSize.x || tid.y >= viewportSize.y) return;
    
    float2 res = float2(viewportSize);
    float2 fragCoord = float2(tid);
    float2 screenPos = (fragCoord / res) * 2.0 - 1.0;
    float3 cameraDir = float3(0.0, 0.0, 0.8);
    float3 cameraPlaneU = float3(1.0, 0.0, 0.0);
    float3 cameraPlaneV = float3(0.0, 1.0, 0.0) * res.y / res.x;
    float3 rayDir = cameraDir + screenPos.x * cameraPlaneU + screenPos.y * cameraPlaneV;
    float3 rayPos = float3(0.0, 2.0 * sin(time * 2.7), -12.0);

    rayPos.xz = rotate2d(rayPos.xz, time);
    rayDir.xz = rotate2d(rayDir.xz, time);

    int3 mapPos = int3(floor(rayPos + 0.5));

    float3 deltaDist = abs(float3(length(rayDir)) / rayDir);
    int3 rayStep = int3(sign(rayDir));
    float3 sideDist = (sign(rayDir) * (float3(mapPos) - rayPos) + (sign(rayDir) * 0.5) + 0.5) * deltaDist;

    bool3 mask;

    for (int i = 0; i < MAX_RAY_STEPS; i++){
        if (getVoxel(mapPos))
            continue;

            mask = sideDist.xyz <= min(sideDist.yzx, sideDist.zxy);
            sideDist += float3(mask) * deltaDist;
            mapPos += int3(float3(mask)) * rayStep; }    

    float3 color;
    if (mask.x) {
        color = float3(0.5);
    }
    if (mask.y) {
        color = float3(1.0);
    }
    if (mask.z) {
        color = float3(0.75);
    }

   // float3 color = .5;
    out.write(float4(color, 1), tid);
}

"""

class MainView: MTKView { 
    
    var time: Float = 0
    
    var viewportSize: simd_uint2 = [0,0]
    
    var commandQueue: MTLCommandQueue!
    var computePass: MTLComputePipelineState!
    
    init(frame:CGRect) {
        
        super.init(frame: frame, device: MTLCreateSystemDefaultDevice())
        
        self.framebufferOnly = false
        self.commandQueue = device?.makeCommandQueue()
        
        var library:MTLLibrary!
        
        do {
            library = try device?.makeLibrary(source: metalFunctions, options: nil)
        } catch{
            print(error)
        }
            
        let shader = library?.makeFunction(name: "shader")
        
        do{
            computePass = try device?.makeComputePipelineState(function: shader!)
        } catch{
            print(error)
        }
        
    }

    required init(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

extension MainView{
    
    override func draw(_ dirtyRect: CGRect) {
        guard let drawable = self.currentDrawable else { return }
        
        viewportSize = [UInt32(drawable.texture.width),
                        UInt32(drawable.texture.height)]
        
        time += 0.01
        
        let commandbuffer = commandQueue.makeCommandBuffer()
        let computeCommandEncoder = commandbuffer?.makeComputeCommandEncoder()
        
        computeCommandEncoder?.setComputePipelineState(computePass)
        computeCommandEncoder?.setTexture(drawable.texture, index: 0)
         computeCommandEncoder?.setBytes(&viewportSize, length: MemoryLayout<simd_uint2>.stride, index: 0)
        computeCommandEncoder?.setBytes(&time, length: MemoryLayout<Float>.stride, index: 1)
        
        var w = computePass.threadExecutionWidth
        let h = computePass.maxTotalThreadsPerThreadgroup / w
        
        var threadsPerThreadGroup = MTLSize(width: w, height: h, depth: 1)
        var threadsPerGrid = MTLSize(width: drawable.texture.width, height: drawable.texture.height, depth: 1)
        var threadgroupsPerGrid = MTLSize(width: drawable.texture.width / w+1, height: drawable.texture.height / h+1, depth: 1)
        computeCommandEncoder?.dispatchThreadgroups(threadgroupsPerGrid, threadsPerThreadgroup: threadsPerThreadGroup)
        
        computeCommandEncoder?.endEncoding()
        commandbuffer?.present(drawable)
        commandbuffer?.commit()
    }
}

let frame = CGRect(x: 0, y: 0, width: 1000, height: 1000)

PlaygroundPage.current.setLiveView(MainView(frame: frame))