Timestamp issues, why timestamp in RTSP is lesser then the one embedded on a frame?

[TrueWodzu]

Question

Hello,

I've created simple video on which I am embedding a timestamp and then I am sending that video to MediaMTX, here is a basic command for encoding each frame:

"appsrc ! videoconvert ! x264enc speed-preset=ultrafast ! rtspclientsink location=rtsp://127.0.0.1:8554/mystream"

Then, I am retrieving each frame via RTSP stream and I am embedding a second timestamp on the image. The second one is a timestamp obtained from RTSP, the one that is added to each packet. The problem is that the second timestamp is incorrect, quite often it is earlier then the one that I've embedded, for example:

The white timestamnp is the one that I've embedded into the frame before sending it to MediaMTX via GStreamer, the blue one is obtained from RTSP. This blue one should never be less than the white one, but it happens quite often. Also the difference between two timestamps is fluctuating very much.

Nothing like this is happening when i am receiving a stream from an actual camera. Then, the difference is always constant and the blue one is always bigger than the white one.

Encoding application and MediaMTX are on the same machine, but MediaMTX is working in docker container. Any help would be much appreciated.