I think it might be your ISP. For example one of my ISPs cut off my SSH connections no matter what I do. They simply dislike hanging SSH connections.
It's just random that your ISP like WebSockets more than long HTTP responses, and it can change in a heartbeat and for most people it will be different. As I said before 99,6% successful networking is an unheard of number for real-time multiplayer games.
I only care about that number, until you proove with hard stats and 350.000 real users from everywhere on the planet that WebSocket has 99,7% success rate, I'm not even going to flinch.