https://mariadb.com/kb/en/mariadb/compression/ and https://mariadb.com/kb/en/mariadb/backup-restore-and-import-clients-xtrabackup/

state that "XtraBackup (at least as of 2.2.12) will not work with MariaDB 10.1 encryption or compression."

I can find no further information on this in Percona, MySQL, MariaDB's documentation or bug trackers or any blogs online.

I think many people may be interested to know why XtraBackup won't work with MariaDB encryption, if that is still the case, and what fixes or workarounds may be available as configuring backups for MariaDB Galera with compression is a not a straightforward path.