Applying texture to a label, but applied instead to the font sheet

[toxic quartz]

Hello I'm trying to apply a "Stamp texture" to a label to make it look like a stamp.

I manage to apply the texture but it seems it doesn't just apply it to my label.
I observe that for similar letters I have the exact same pattern (see the letter "C" or "E" on my second screenshot), so it seems that the texture isn't applied to my label but to the font sheet.

My shader code is the following:

shader_type canvas_item;
uniform sampler2D alpha_texture;

void fragment() {

    if (COLOR.a != 0.){
        COLOR  = vec4(COLOR.r, COLOR.g, COLOR.b, 1.- texture(alpha_texture, UV).x - 0.2) ;
    }
}

Image 1 is my label without the shader applied
Image 2 is my label with the shader applied
Image 3 is the texture that I want to apply

Thank you in advance

Applying texture to a label, but applied instead to the font sheet

[languid socket]

I've always struggled with this as well and end up putting the label in a viewport and applying the shader on the resulting texture

[gritty comet]

Yeah, or any node that takes all the letters as one texture