When using ite incremental backups of tablespaces are full backups and compression

We are using Mysql 5.7.18 with keyring plugin. We use the xtrabackup version 2.4.7 the parameters for backup is as follows
xtrabackup --backup --user=root --password=x xxxxxxxxxxxx --keyring-file-data=/opt/mysql/admin/CARD/cfg/keyring --server_id=1 --compress --parallel=4 --compress-threads=4 --no-timestamp --target-dir=/opt/mysqlbackup/CARD/2017-04-24-09-54-full

Even when using incremental backups the delta is a full backup of the encrypted table.

This is a huge problem since the database is 665GB big and we take daily backups