client is not initiating the WebSocket closing handshake
As per RFC https://tools.ietf.org/html/rfc7395 section 3.6,
Either the server or the client may close the connection at any time. Before closing the connection, the closing party is expected to first close the XMPP stream (if one has been opened) by sending a message with the element, qualified by the "urn:ietf:params:xml:ns:xmpp-framing" namespace. The stream is considered closed when a corresponding element is received from the other party, and the XMPP session is ended.
To then close the WebSocket connection, the closing party MUST initiate the WebSocket closing handshake (see Section 7.1.2 of [RFC6455]).
While user disconnects, Jaxmpp is terminating stream by sending "". It doesn't send close the Websocket connection which is MUST as per RFC.