rtpsession: NACKs are not being sent

GStreamer version: 1.21.0.1, commit: 8aeb8ea5

Hi, I'm trying to enable NACK between Pion (sender) and GStreamer (receiver).

My pipeline looks like this:

  udpsrc = MakeElement("udpsrc");
  rtpsession = MakeElement("rtpsession");
  auto rtprtxreceive = MakeElement("rtprtxreceive");
  auto rtpjitterbuffer = MakeElement("rtpjitterbuffer");
  auto rtph264depay = MakeElement("rtph264depay");
  auto h264parse = MakeElement("h264parse");
  auto avdec_h264 = MakeElement("avdec_h264");
  tee = MakeElement("tee");

  auto udpsink = MakeElement("udpsink", "rtcp_udpsink");

  recv_rtp_sink = gst_element_request_pad_simple(rtpsession, "recv_rtp_sink");
  auto udpsrc_src = gst_element_get_static_pad(udpsrc, "src");
  gst_pad_link(udpsrc_src, recv_rtp_sink);
  gst_object_unref(udpsrc_src);

  auto recv_src = gst_element_get_static_pad(rtpsession, "recv_rtp_src");
  auto rtprtxreceive_sink = gst_element_get_static_pad(rtprtxreceive, "sink");
  gst_pad_link(recv_src, rtprtxreceive_sink);
  gst_object_unref(recv_src);
  gst_object_unref(rtprtxreceive_sink);

  send_rtcp_src = gst_element_request_pad_simple(rtpsession, "send_rtcp_src");
  auto udpsink_sink = gst_element_get_static_pad(udpsink, "sink");
  gst_pad_link(send_rtcp_src, udpsink_sink);
  gst_object_unref(udpsink_sink);

  gst_element_link_many(rtprtxreceive,
                        rtpjitterbuffer,
                        rtph264depay,
                        h264parse,
                        avdec_h264,
                        tee,
                        NULL);

  g_object_set(udpsrc, "port", 0, NULL);
  g_object_set(udpsink,
               "host", sink_hostname.c_str(),
               "port", sink_rtcp_port,
               "sync", false,
               "async", false, NULL);
  g_object_set(rtpjitterbuffer, "do-retransmission", true, NULL);

...

But on Pion side the only RTCP packets I'm receiving are ReceiverReport and Source Description:

2022/08/18 17:19:24 common.go:105: [debug] ReceiverReport from 4a2ce4ea
        SSRC            Lost    LastSequence
        9e484bbf        40/60   18680
        Profile Extension Data: []

2022/08/18 17:19:24 common.go:105: [debug] Source Description:
        4a2ce4ea: [{CNAME user539376885@host-6e049b7d} {TOOL GStreamer}]

What am I doing wrong?

I've checked recv_rtp_src pad and there are GstRTPRetransmissionRequest events incoming.

So the line that actually fails is here, not sure why it happens though