Shaders on mobile not working

I've recently started learning how to write shaders and how to apply them to a camera
i tried to use a few of my custom shaders on android but they just returned a black screen or flickering black/white screen with a few pixel being the other shade, or rarely a random colour
even the default invert preset does it

i can only think of the tex2d function not returning the colour of the screen pixel or _mainTex not being set to the rendered image of the camera (it wouldnt explain the rare random coloured pixels tho)

this is one of my custom shaders thats acting weird on android for example

Code (CSharp):

1. Shader "Camera/circleInvert"
2. {
3.     Properties
4.     {
5.         _MainTex ("Texture", 2D) = "white" {}
6.         _speed ("Circle Speed", float) = 1
7.         _size ("Circle Size", float) = 1
8.     }
9.     SubShader
10.     {
11.         // No culling or depth
12.         Cull Off ZWrite Off ZTest Always
13.  
14.         Pass
15.         {
16.             CGPROGRAM
17.             #pragma vertex vert
18.             #pragma fragment frag
19.  
20.             #include "UnityCG.cginc"
21.  
22.             struct appdata
23.             {
24.                 float4 vertex : POSITION;
25.                 float2 uv : TEXCOORD0;
26.             };
27.  
28.             struct v2f
29.             {
30.                 float2 uv : TEXCOORD0;
31.                 float4 vertex : SV_POSITION;
32.             };
33.  
34.             v2f vert (appdata v)
35.             {
36.                 v2f o;
37.                 o.vertex = UnityObjectToClipPos(v.vertex);
38.                 o.uv = v.uv;
39.                 return o;
40.             }
41.  
42.             sampler2D _MainTex;
43.             float _size;
44.             float _speed;
45.  
46.             fixed4 frag (v2f i) : SV_Target
47.             {
48.                 float2 uvMid = i.uv - 0.5;
49.                 float dist = sqrt(uvMid.x * uvMid.x + uvMid.y * uvMid.y);
50.                 float timing = ( sign( sin(_Time[1] * _speed+dist * _size)) + 1)/2.;
51.                
52.                 i.uv.y = timing * i.uv.y + (1-timing) * (1-i.uv.y);
53.  
54.                 fixed4 col = tex2D(_MainTex, i.uv);
55.                 col.rgb = timing * col.rgb + (1-timing)*(1-col.rgb);
56.                 return col;
57.             }
58.             ENDCG
59.         }
60.     }
61. }

does anyone know why it wont work on android?

 

this just returns a black screen

as said in my question, even the default template (which is just colour inversion) returns a flashing screen with random pixel not being inverted and some rare pixels even having a random color
if needed i can upload screenshots of the different expectations and outcomes

tried with unity versions 2019.3.0 and 2019.1.6

 

ive not known of logcat yet but this is the error: (the shader is still the above with only get colour and return)

 

i have a monobehavior class in which i use graphic.blit in onRenderImage
code(it works normaly in editor and windows builds):

Code (CSharp):

1. [ExecuteInEditMode]
2. public class CameraMaterials : MonoBehaviour
3. {
4.     [SerializeField]
5.     List<Material> materials = new List<Material>();
6.     private void OnRenderImage(RenderTexture source, RenderTexture destination)
7.     {
8.         foreach(Material shader in materials)
9.         {
10.             Graphics.Blit(source, source, shader);
11.         }
12.         Graphics.Blit(source, destination);
13.     }
14. }