This endpoint is heavily cached and will take some time to update to show people who have joined (or remove) people whom have left.
And there is no reliable way to get the current list of viewers, the one you are calling only returns people in chat.
Followers only mode has no bearing on showing up/not showing up
That /chatters JSON only returns people whom are in chat, not people whom are watching the stream.
Yeah it’s a undocumented endpoint. There is no endpoint that Twitch officially provides to get whom is watching a stream (or whom is in a chat of a stream).
The closest we get is Drops but Drops is locked to Games Developers.
You could achieve what you are after using a Twitch Video extension, but that will exclude people on mobile and non extension enabled platforms (such a console)
We have a uservoice at
requesting Twitch to add an officially supported endpoint for us