The documentation of the method Logger.throwing(Throwable) is not mentioning at which level it is logging at.
It appears AbstractLogger is logging at level ERROR, but without it being explicitly mentioned, I think expecting it to be logged at DEBUG might also be a reasonable expectation.
LOG4J2-1003 implements it to log at debug level, and based on what the throwing documentation suggests as use case, AbstractLogger should be logging at debug level:
Having an exception which is thrown additionally be logged as error will result in duplicate error messages in the log file (unless the exception handling is broken), one from the throwing call and a second one from the method handling that exception.
Therefore throwing should in my opinion only log at DEBUG level, similar to how java.util.logging is doing it.