This is actually a bug in the underlying Java-WebSocket library used by this project:
When a client uses a client certificate for authentication, a bug in Java-WebSocket's SSLSocketChannel2.java causes the TLS handshake to get stuck.
Reference: TooTallNate/Java-WebSocket#1500
As a result, to implement an OCPP Central System / CSMS for security profile 3 (TLS with client certificate authentication) using this library, a bugfixed SSLSocketChannel2.java replacement in addition to copies of WebSocketServerFactory and WssFactoryBuilder which use this replacement must be created and an instance of that WssFactoryBuilder passed to [MultiProtocol]JSONServer.enableWSS();