GLSL shader that draws an outline around my object

Question

I'm trying to make a GLSL shader. The shader should display a red outline around my character. The shader works, but on some specific characters there are red lines, which I don't know how to fix

This is how my shader fragmetn looks:

uniform mat4 u_Color;
varying vec2 v_TexCoord;
varying vec2 v_TexCoord2;
uniform sampler2D u_Tex0;

const float ALPHA_TOLERANCE = 0.01;

void main()
{
    vec4 baseColor = texture2D(u_Tex0, v_TexCoord);
    vec4 texcolor = texture2D(u_Tex0, v_TexCoord2);
    if(texcolor.r > 0.9) {
        baseColor *= texcolor.g > 0.9 ? u_Color[0] : u_Color[1];
    } else if(texcolor.g > 0.9) {
        baseColor *= u_Color[2];
    } else if(texcolor.b > 0.9) {
        baseColor *= u_Color[3];
    }

    vec4 pixel1 = texture2D(u_Tex0, vec2(v_TexCoord.x + 0.001, v_TexCoord.y));
    vec4 pixel2 = texture2D(u_Tex0, vec2(v_TexCoord.x - 0.001, v_TexCoord.y));
    vec4 pixel3 = texture2D(u_Tex0, vec2(v_TexCoord.x, v_TexCoord.y + 0.001));
    vec4 pixel4 = texture2D(u_Tex0, vec2(v_TexCoord.x, v_TexCoord.y - 0.001));

    bool neighbourColor = pixel4.a > ALPHA_TOLERANCE || pixel3.a > ALPHA_TOLERANCE || pixel2.a > ALPHA_TOLERANCE || pixel1.a > ALPHA_TOLERANCE;

    if (baseColor.a < ALPHA_TOLERANCE && neighbourColor) {
        baseColor.rgb = vec3(1.0, 0.0, 0.0);
        baseColor.a = 0.7;
    }
    
    gl_FragColor = baseColor;
    if(gl_FragColor.a < 0.01) discard;
}

This is how my vertex shader looks:

attribute vec2 a_Vertex;
attribute vec2 a_TexCoord;
uniform mat3 u_TextureMatrix;
varying vec2 v_TexCoord;
varying vec2 v_TexCoord2;
uniform mat3 u_TransformMatrix;
uniform mat3 u_ProjectionMatrix;
uniform vec2 u_Offset;

void main()
{
    gl_Position = vec4((u_ProjectionMatrix * u_TransformMatrix * vec3(a_Vertex.xy, 1.0)).xy, 1.0, 1.0);
    v_TexCoord = (u_TextureMatrix * vec3(a_TexCoord,1.0)).xy;
    v_TexCoord2 = (u_TextureMatrix * vec3(a_TexCoord + u_Offset,1.0)).xy;
}

The shader works like this: it checks if the color is transparent, if it is, then it checks if a neighboring pixel has a real "color", if it is then it's replaced with red. But for some reason there are some random red lines outside of my character. There isn't any "real" colors next to those lines, so I don't understand why it's making those pixels red. Example of a bugged character