GStreamer 1.21. qtmux produces garbage on recording plain H.264 when resolution changes

Question

This is the pipeline:

webrtcbin name=webrtcbin bundle-policy=max-bundle! rtph264depay ! h264parse ! qtmuxfragment-duration=1000 ! filesink location=/home/ubuntu/recordings/abcde/test.mkv

Modern browsers vary resolution and bitrate during the startup phase of a session. It is not uncommon to see 640x360 for a while, followed by 960x540 up to maybe 1280x720 later in a H.264 stream.

Trying to record this to disk requires the use of either mp4mux or qtmux. Both did break up the recording on the first resolution change in versions prior GStreamer 1.20. Practically this was the end of the recording when the first change from 640x360 to 960x540 happened.

Meanwhile (after 1.20) the qtmux survives the renegotiation with upstream, but the downstream does not contain the required (and updated) SPS/PPS. Just the IDR frame (initiated by the browser) is sent downstream. In the end this means, that after the first resolution change there is only garbage in the output file.

Any idea, how this could be changed?