It seems https://github.com/jenkinsci/jenkins/commit/6a2d94bfbe9fca4d020f513025100b66872a1877 is involved on the issue.

• Now agents using this kind of launcher implementation are created setting webSocket to false by default.

• You cannot modify it.

• As they are created to false by default, the configuration block is hidden https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/slaves/JNLPLauncher.java#L208

As it is set to false, if you try to connect without adding the webSocket argument it uses the TCP port.