QMediaRecorder captures video at a much worse quality on Linux

This problem is currently only observed on Linux (Ubuntu 22.04 and reportedly RHEL8). I've also tried Windows and macOS, and reproducer works well on those platforms.

Anyway, build the attached reproducer against 6.8.3 or 6.9.0. The design is that it's going to pick up the .mp4 (one may need to change the path in QUrl::fromLocalFile()), feed it into QMediaPlayer and eventually QVideoSink. That QVideoSink is going to provide video frames to QVideoFrameInput which will be used in a QMediaCaptureSession and recorded by QMediaRecorder. Finally, it's going to output a .mp4 file next to the binary via QFile.

The problem is that the output has a much worse quality than the original video. The frames seem to be badly pixelated. One can also see from the file size, the the output is around 600 kb while the original video is about 2mb.

BTW, formatting has been explicitly specified:

    mediaFormat.setVideoCodec(QMediaFormat::VideoCodec::MPEG4);
    mediaFormat.setFileFormat(QMediaFormat::FileFormat::MPEG4);
    recorder.setMediaFormat(mediaFormat);
    recorder.setEncodingMode(QMediaRecorder::ConstantQualityEncoding);
    recorder.setQuality(QMediaRecorder::VeryHighQuality);

That does not make things better though.

I also uploaded the output video from my end. One can use that as a "reference" of a "bad quality".