WebRTC recvonly when answering with audio to Chrome/Firefox

When adjusting the gstwebrtc-demo project to allow browser to make the offer, webrtcbin answers with recvonly for audio. This only happens when creating an answer to the browser. When the gstreamer side generates an offer, the browser answers with sendrecv and both sides work.

Here is a pastebin of an audio-only session log (GST_DEBUG="*webrtc*:7"), where the gstreamer side answers to the browser with recvonly. https://pastebin.com/xdWk0ZKA

What is clear from the log is that webrtcbin notices the VP8 and OPUS caps on its sink pads. However, no audio is negotiated to send to the browser.

I have forked the demos here: https://github.com/Climax777/gstwebrtc-demos/tree/recvonlybug in the recvonlybug branch. I've only used the sendrecv demo at this stage since it involves the browser.