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:

https://github.com/MariaDB/server/blob/370a2cbe96e026fdb8966d8e58e7c93f75597cb8/scripts/wsrep_sst_xtrabackup-v2.sh#L335

Would this be considered a bug, or is this acceptable behavior?