isomp4mux can stall pipeline waiting for video

When using isofmp4mux, we observe pipeline stalls in certain scenarios.

We have audio & video path, feeding a video and audio tee. These tees then feed the two bins downstream. One bin has queue + flvmux + rtmp2sink. Other has queue + fmp4mux + awss3sink. flvmux latency is set to 100ms. Fragment duration is set at 2 seconds and queue sizes have max-size-time=5s. (See attached images for details).

Either of the two bins might be attached dynamically at runtime depending on user request for streaming or recording. We observe a stall where the RTMP stream completely freezes just after attaching the recording bin. A section of the dot graph, captured just around the time of freeze is attached. The problem seems to go away if queue sizes are increased with max-size-time=15s. However, if chunk-duration is set, the problem is even more easily reproducible every time even with max-size-time=15s. This goes away if we set max-size-time/buffers/bytes=0.

Occasionally we also see Don't have a complete GOP for the first stream on timeout in a live pipeline messages from the video sink pad of isofmp4mux. From the dot graphs, it would seem that isofmp4mux might stall waiting for video keyframe?.

If it helps, upstream to the video tee, we have x264enc, capsfilter and h264parse. For x264enc, we set bitrate=5000, key-int-max=60, speed-preset=veryfast and tune=zerolatency.