Grayscale video file corrupted in OpenCV3.3, MJPG codec - c++

Question

My setup is using a logitech c920, raspberry pi 3, and the latest opencv 3.3. I am showing in screen and writing in a file the camera stream. The only processing I am doing is converting video to grayscale. In the screen all show good, but the file is corrupted (see https://i.stack.imgur.com/QHrQb.png).
Noticeable, the video is well recorded in XVID, and also in MJPG if the original color image is selected instead.
UPDATE: I tested the same code in os-x and the same error happens, also with opencv3.3
Any advice welcome :)
This is the code:
#include "opencv2/opencv.hpp"
#include <iostream>
using namespace std;
using namespace cv;
int main() {
int FPS = 10;
int nframes;
double start, now;
VideoCapture vcap(0);
if(!vcap.isOpened()) {
cout << "Error opening video stream or file" << endl;
return 0;
}
vcap.set(CV_CAP_PROP_FOURCC,CV_FOURCC('M','J','P','G'));
int frame_width = vcap.get(CV_CAP_PROP_FRAME_WIDTH);
int frame_height = vcap.get(CV_CAP_PROP_FRAME_HEIGHT);
VideoWriter video("out.avi",CV_FOURCC('M','J','P','G'), FPS, Size(frame_width,frame_height), false);
namedWindow("Main",CV_WINDOW_AUTOSIZE); //create a window called
// Start time
start = (double)getTickCount();
for(;;) {
Mat frame, gray;
vcap >> frame;
cvtColor(frame, gray, cv::COLOR_BGR2GRAY);
video << gray;
imshow("Main", gray);
char c = (char)waitKey(2);
if( c == 27 ) break;
++nframes;
if (nframes==100) {
now = (double)getTickCount();
cout << "FPS: " << ++nframes/(now-start)*getTickFrequency() << endl;
start = now;
nframes = 0;
}
}
video.release();
return 0;
}