When I close the connection with a web socket by ws.close() on a client side. The disconnected Azure Function is not triggered. Please help! My binding is:

{
"bindings": [{
"type": "webPubSubTrigger",
"direction": "in",
"name": "data",
"hub": "hubName",
"eventName": "disconnected",
"eventType": "system"
}]
}