Epic have given us an official workaround. You need to add this line to the webRtcPlayer.js file in the signalling server.
offer.sdp = offer.sdp.replace(/(a=extmap-allow-mixed)\r\n/gm, “”);
This is around line 125, so you end up with this block (apologies for bad formatting):
handleCreateOffer = function (pc) {
pc.createOffer(self.sdpConstraints).then(function (offer) {
pc.setLocalDescription(offer);
if (self.onWebRtcOffer) {
// (andriy): increase start bitrate from 300 kbps to 20 mbps and max bitrate from 2.5 mbps to 100 mbps
// (100 mbps means we don’t restrict encoder at all)
// after we setLocalDescription because other browsers are not c happy to see google-specific config
offer.sdp = offer.sdp.replace(/(a=fmtp:\d+ .level-asymmetry-allowed=.)\r\n/gm, “$1;x-google-start-bitrate=10000;x-google-max-bitrate=20000\r\n”);
offer.sdp = offer.sdp.replace(/(a=extmap-allow-mixed)\r\n/gm, “”);
self.onWebRtcOffer(offer);
}
},
function () { console.warn(“Couldn’t create offer”) });
}