Fixed by copying the vertex shader code from the standard Unity sprite shader.
@ -64,8 +64,12 @@
{
v2f OUT;
OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
OUT.texcoord = TRANSFORM_TEX(IN.texcoord, _MainTex);
OUT.texcoord = IN.texcoord;
OUT.color = IN.color * _Color;
#ifdef PIXELSNAP_ON
OUT.vertex = UnityPixelSnap (OUT.vertex);
#endif
return OUT;
}