We have the exact same issue however in Ver5, but we manage to track it down partly. Dont know if this might be helpful in your case, but our STUCK visitors seems to be whenever we have one moving from our HTTP frontend to our HTTPS backend. this doesnt call a disconnect and the session is still active. Due to that we move the HTTP -> HTTPS for simplifying our basket handling.

 

Perhaps some light on the mysterious growing queue.

 

/Rikard