There is the Websocket PING (part of the protocol), and there is Twitch’s PubSub PING. They are not related.
Twitch has decided to not use the built-in PING/PONG but requires applications to send it themselves as a normal message.
To fix disable _ws.EnableAutoSendPing = true; and implement your own periodic ping as specified by the docs: https://github.com/justintv/Twitch-API/tree/master/PubSub#connection-management