Why does my ARCore AugmentedFace mask flicker?

I was experementing with a tutorial project from here. I wanted the application to fill the user face with just one color. So I wrote new vertex and fragment shaders for this.

my_shader.vert

uniform mat4 u_ModelViewProjection;

attribute vec4 a_Position;

void main() {
   gl_Position = u_ModelViewProjection * a_Position;
}

my_shader.frag

precision mediump float;

void main() {
    gl_FragColor = vec4(1.0, 0.0, 1.0, 1.0);
}

The application now draws one-color mask on the user face as I wanted. But this mask is flickering. I don't know what to do, because my shaders are very simple and I didn't change anything else in project.