Libgdx - shader ignores mix / alpha in the first frame

I am creating a libgdx grid using a shader with alpha less than 1.0.

When rendering the first frame , the alpha value set in the shader is ignored and displayed as 1.0. All of the following rendering frames are beautiful.

The same thing happened to me in the previous lines and forms of drawing a project using glDrawArrays, and I have not yet found a solution.

Code in the libgdx render () loop:

    Gdx.gl20.glClearColor(0, 0, 0, 1);
    Gdx.gl20.glClear(GL20.GL_COLOR_BUFFER_BIT | GL20.GL_DEPTH_BUFFER_BIT);

    Gdx.gl20.glEnable(GL20.GL_BLEND);
    MeshManager_colorFill.meshShader.begin();
    MeshManager_colorFill.meshShader.setUniformMatrix("u_worldView", Snappr.camMatrix);

    meshArray_colorFill.render(MeshManager_colorFill.meshShader, GL20.GL_TRIANGLE_FAN, 0, meshArray_colorFill.get(i).getNumVertices()); 

    MeshManager_colorFill.meshShader.end();
    Gdx.gl20.glDisable(GL20.GL_BLEND);

My shader (compiled into create () {}):

  public static final String meshVertexShader_colorFill =

                    "attribute vec2 a_position;    \n" + 
                    "uniform mat4 u_worldView;\n" + 
                    "void main()                  \n" + 
                    "{                            \n" + 
                    "   gl_Position =  u_worldView * vec4(a_position.xy, 0, 1);  \n"      + 
                    "}                            \n" ;

  public static final String meshFragmentShader_colorFill =

                    "precision mediump float;\n" +  
                    "void main()                                  \n" + 
                    "{                                            \n" + 
                    "  gl_FragColor = vec4(1,1,1,0.2);\n" +
                    "}";

How to make the first frame the way it should be?

thank