In Unity, some devices do not clear properly in areas where no viewport is rendering. Adding this to anycamera fixes it (check the depth too)

ClearFullScreen.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ClearFullScreen : MonoBehaviour {

    private void OnPreCull()
    {
        GL.Clear(true,true,Color.black,0);//1);
    }
}