v4l2src: pts of the first frame is not correct
I'm running the pipeline: gst-launch-1.0 -e v4l2src ! x264enc ! matroskamux ! filesink location=dump.mkv with this camera: https://www.logitech.com/en-roeu/product/brio-stream-4k-hd-webcam. I'm capturing the video stream from the camera, encoding and muxing it to a file. The camera captures with 30fps. The pts of the first frame is not always correct (approximately 1 out of 5-15 times):
The video file is the camera capture of the timer: https://www.timeanddate.com/stopwatch/
the PTS of the first 3 frames are:
v4l2src0:src pts: 961ms (961354643 ns)
v4l2src0:src pts: 1185ms (1185390192 ns)
v4l2src0:src pts: 1209ms (1209376552 ns)
but according to the video file the diff between the first and the second frame is 25ms.
Is it expected behavior of the v4l2src element or is it a bug?