Weird shader behaviour on just one Android Device

Hi there,

I wrote a custom shader to simulate a camera circular close effect with a custom blur. Basically it works well on IOS, on the Editor, and on some Android devices, but on a PhonePad S501, it's like the coordinates of the distance between the pixel and the center of the screen are reported wrong, making the filter starting at 50% of it's effect and then blurring in the opposite direction after the 50% of time... I think this has something to do with the differences between the platforms and the GFX card but I'm not sure where to begin to debug it.
I'm attaching the shader:

Shader "Camera/Camera Close Effect" {
Properties {
_MainTex ("Base (RGB)", 2D) = "white" {}
}

SubShader
{
Pass
{
ZTest Always Cull Off ZWrite Off

CGPROGRAM
#pragma vertex vert
#pragma fragment frag

#include "UnityCG.cginc"

uniform sampler2D _MainTex;
uniform float _radius;
uniform float _blur;

struct v2f {
float4 pos : SV_POSITION;
float2 uv : TEXCOORD0;
};

v2f vert (appdata_img v)
{
v2f o;
o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
o.uv = v.texcoord.xy ;

return o;
}

float4 frag (v2f i) : SV_Target
{
float4 sampledColor = tex2D(_MainTex, i.uv);
float2 screen = _ScreenParams.xy;
float2 screenPosition = i.uv * screen.xy;
float2 screenCenter = screen / 2.0f;
float ar = screen.y / screen.x;
float dist = distance(screenPosition, screenCenter);
float normalizedRadius = (screen.x * _radius * ar) * (1.0f + _blur);
float normalizedBlur = screen.x * _blur;
if (screen.y > screen.x) {
normalizedRadius = screen.y * _radius / ar * (1.0f + _blur);
normalizedBlur = screen.y * _blur;
}

if (dist < normalizedRadius- normalizedBlur) {
return sampledColor;
} else {
float offset = normalizedRadius - dist;
float amount = offset/normalizedBlur;
float4 mask = float4(amount, amount, amount, 1.0f);
sampledColor.rgb = lerp(sampledColor.rgb, lerp(mask.rgb, sampledColor.rgb, amount), mask.a);
return sampledColor;
}
}
ENDCG
}
}

Fallback off

}