VideoWriter not writing anything

Description

cv2.VideoWriter is creating a file but not writing anything in the video

Environment

Ubuntu 25.04
Python 3.13.3
Open CV 4.13.0

How to recreate

Create main.py with the file content.
Create a video file named video.mp4
mkdir .venv
python -m venv .venv
. .venv/bin/activate
pip install opencv-contrib-python
python main.py
And I get the following output with a corrupted output.mp4 file:

OpenCV Version:
4.13.0
codec
1983148141
shape
(1576, 770)
fps
24.0

However, if I run
ffmpeg -i output.mp4 out.mp4
out.mp4 works

File

import cv2
import time

video = cv2.VideoCapture("video.mp4")

fps = video.get(cv2.CAP_PROP_FPS)
shape = (int(video.get(cv2.CAP_PROP_FRAME_WIDTH)), int(video.get(cv2.CAP_PROP_FRAME_HEIGHT)))
fourcc = cv2.VideoWriter_fourcc(*"mp4v")
writer = cv2.VideoWriter("output.mp4", fourcc, fps, shape)

print("OpenCV Version:")
print(cv2.__version__)
print("codec")
print(fourcc)
print("shape")
print(shape)
print("fps")
print(fps)

time.sleep(1.0)

while True:
    ret, frame = video.read()

    if not ret or frame is None:
        break

    if frame.ndim == 2:
        frame = cv2.cvtColor(frame, cv2.COLOR_GRAY2BGR)

    writer.write(frame)


video.release()
writer.release()

Start by running the supplied main.py with video.mp4 in the stated Python and OpenCV environment, then inspect the generated output.mp4 and the VideoWriter setup. Compare it with the working ffmpeg-converted out.mp4; done means the script itself produces a playable output.mp4 without requiring a conversion step.