lock-ddl-per-table option not working as expected in Xtrabackup

I've recently been getting the following errors on some of my Percona XtraBackup backups: InnoDB: An optimized (without redo logging) DDL operation has been performed. All modified pages may not have been flushed to the disk yet. However, it was my understanding that this should be avoided by including the --lock-ddl-per-table option. The script I use is as follows: xtrabackup ${MySQLConnectionString} --backup --parallel=${CPUCount} --compress --compress-threads=${CPUCount} --throttle=${throttle} --slave-info --extra-lsndir="${LogsDir}" --target-dir="${CurrentDir}" --lock-ddl-per-table xtrabackup ${MySQLConnectionString} --backup --parallel=${CPUCount} --compress --compress-threads=${CPUCount} --throttle=${throttle} --slave-info --extra-lsndir="${LogsDir}" --target-dir="${NewDir}" --incremental-basedir="${BaseDir}" --lock-ddl-per-table My Databases are all MySQL 5.7.28, and they are slave databases. They are all running on Debian 9, and using PXB version 2.4.16. Have I misunderstood the usage of this option?