Video output not (yet) flushed

I have a completely working program reading frames from a camera and writing video to disk.
Each 20 frames I create a new file on disk.

While the program is active, I see files being created but when I try to display them with VLC nothing is presented.

Only when I terminate the program (with ctrl-C), the files can be displayed using VLC.
It is if the file is not completely flushed to disk…

I want to display the stored file even when the program is still running.
What do I do wrong?

Platform: Raspberry Pi 4

This is the source-code:

#include <stdio.h>
#include <opencv2/opencv.hpp>
#include <fstream>
#include <iostream>
#include <opencv2/core/ocl.hpp>
#include <stdlib.h>

#include <signal.h>


using namespace cv;
using namespace std;

const size_t mv_width  = 300;
const size_t mv_height = 300;
bool         mv_quit   = false;


void my_handler(int signum){    // detects a ctrl-C
    mv_quit = true;
}

int main(int argc,char ** argv)
{
    Mat         frame;
    int         lv_count = 0;
    VideoWriter play;

    signal (SIGINT,my_handler);         // Ctrl-C signal handler

    VideoCapture cap(0);
    if (!cap.isOpened()) {
        cerr << "ERROR: Unable to open the camera" << endl;
        return 0;
    }

    cout << "Start grabbing, press ctrl-c to terminate" << endl;

    while(1){
        cap >> frame;
        if (frame.empty()) {
            cerr << "End of movie" << endl;
            break;
        }

        resize(frame, frame, Size(mv_width,mv_height));

        if ((lv_count % 20) == 0) {  // every 0 frames I want a new file
            play.release();         // close and flush file to disk

            cout << "CREATED FILE CAN NOT BE DISPLAYED WITH VLC YET" << endl;

            char lv_buffer[40];
            sprintf(lv_buffer,"/home/pi/Videos/CAT%d.avi", lv_count);
            play.open(lv_buffer, cv::VideoWriter::fourcc('M','J','P','G'), 4, Size(mv_width,mv_height));
        }
        play.write(frame);
        lv_count++;
        cout << lv_count << endl;

        if (mv_quit == true) break;
    }


    cap.release();
    play.release();

    cout << "ONLY AFTER THIS POINT THE CREATED FILES CAN BE DISPLAYED WITH VLC" << endl;
    return 0;
}