Some DDL statements can lead to inconsistent backups unless the server has innodb_log_optimize_ddl=OFF set. See
MDEV-16791 for more information.
Maybe Mariabackup should write a warning if a backup is taken when innodb_log_optimize_ddl=OFF is not set? i.e. something like this?:
[Warning] The server does not have innodb_log_optimize_ddl set to OFF. If certain DDL statements are executed during the backup, then the backup can be inconsistent.