We've had two separate reports of this now, same issue both times: meta:293657 and meta:293622.

I've checked the logs and the __html attribute that should contain the HTML rendered by our JS was sent with no content in it. I haven't been able to work out why.

One workaround may be to check on the server side that the generated HTML is longer than the Markdown sent (I can't think of any cases where the Markdown would be longer than the HTML), and render the Markdown server-side if it's not.