We have been chasing this down for a while. This issue reappeared two nights back. I was noticing that it was because the per-message size was in the 300K to 500K range.
So I tried to reduce our payload, and our issue went away.
Digging in, I noticed that the moment I disable PerMessageDeflateHandshake() extension the error goes away even at the large batch size. So this points to some bug in that Websocket Extension.