As best as I can tell – It’s turned out to be difficult to get definitive info back from the streamers – but looking over the data feed and keeping an eye on traffic, it seems there may be a Twitch-side bug that manifests itself when there are large numbers of visitors to a particular stream using a Full Screen Overlay.
At some point the Overlay becomes unreliable – it’s installed, but it only sometimes loads / sometimes doesn’t. It seems to have nothing to do with the streamer changing settings / installation of the Overlay. Perhaps.
We seem to have no problem where traffic is under 5,000 viewers per day (though I can’t be sure – perhaps even at those numbers the Overlay doesn’t load for X percentage of viewers.
Above 10 or 15,000/day we’re seeing it. On a day with 400,000 visitors we saw 60,000 loads.