In MariaDB 10.2, some DDL statements can lead to inconsistent backups unless the server has innodb_safe_truncate=ON set. See
MDEV-13564 for more information.
Maybe Mariabackup in MariaDB 10.2 should write a warning if a backup is taken when innodb_safe_truncate=ON is not set? i.e. something like this?:
[Warning] The server does not have innodb_safe_truncate set to ON. If certain DDL statements are executed during the backup, then the backup can be inconsistent.