If the syslog option of mysqld_safe and the log_error mysqld option are both set, the log_error option takes precedence, so mysqld will log its log entries to its own log file, not to syslog.
However, wsrep_sst_xtrabackup-v2 does not have the same behavior. If the syslog option of mysqld_safe is enabled, it will log to the syslog, regardless of whether log_error is also enabled. You can see the responsible check here:
Would this be considered a bug, or is this acceptable behavior?