I don't see a reason why close(CloseReason) shouldn't be allowed on a closed Session while close() is perfectly legal.
EE 7 Javadoc for Session says:
Once the session is closed, it is no longer valid for use by applications. Calling any of its methods (with the exception of the close() methods) once the session has been closed will result in an IllegalStateException being thrown.
"close() methods" is plural (and "the session" is singular), so I conclude it must mean both close methods on a single object, namely close() and close(CloseReason).