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After some research and experiment, finally i find the light, here it is. you just need to get the image back to the default type. like this :

for (Rect r : faces){
        Core.rectangle(mRgba, r.tl(), r.br(), new Scalar(0, 255, 0, 255), 3);
        Mat roi = new Mat();
        roi = mRgba.submat(r);

        Mat gray = new Mat();
        Imgproc.cvtColor(roi, gray, Imgproc.COLOR_RGB2GRAY,1);

        Mat gray_f = new Mat();
        gray.convertTo(gray_f, CvType.CV_32F, 1.0/255, 0);

        Mat final = new Mat();
        gray_f.convertTo(final, CvType.CV_8U, 255, 0);

        Imgproc.cvtColor(final, roi, Imgproc.COLOR_GRAY2RGB,0);
        Imgproc.cvtColor(roi, roi, Imgproc.COLOR_RGB2RGBA,0);
        Mat kotak = new Mat();
        kotak = mRgba.colRange(r.x, r.x+r.width).rowRange(r.y, r.y+r.height);
        roi.copyTo(kotak);
    }     }

This is the Queen of This Answer :

        Mat final = new Mat();
        gray_f.convertTo(final, CvType.CV_8U, 255, 0);