When running SST with aws_key_management_log_level enabled:

200326 13:05:39 Connecting to MySQL server host: localhost, user: root, password: set, port: not set, socket: /var/lib/mysql/mysql.sock
Using server version 10.1.44-MariaDB
Loading encryption plugin
Encryption plugin parameter : '--aws_key_management_key_spec=AES_256'
Encryption plugin parameter : '--aws_key_management_log_level=Info'
Encryption plugin parameter : '--aws_key_management_master_key_id=alias/mariadb_key'
Encryption plugin parameter : '--aws_key_management_region=us-east-1'
Encryption plugin parameter : '--aws_key_management_request_timeout=0'
Encryption plugin parameter : '--aws_key_management_rotate_key=0'
--innobackupex based on MariaDB server 10.1.44-MariaDB Linux (x86_64)
mariabackup: uses posix_fadvise().
mariabackup: cd to /var/lib/mysql/
mariabackup: open files limit requested 200000, set to 68575
mariabackup: using the following InnoDB configuration:
mariabackup: innodb_data_home_dir =
mariabackup: innodb_data_file_path = ibdata1:12M:autoextend
mariabackup: innodb_log_group_home_dir = ./
mariabackup: innodb_log_files_in_group = 2
mariabackup: innodb_log_file_size = 4294967296
mariabackup: using O_DIRECT
200326 13:05:39 >> log scanned up to (9219062449)
mariabackup: Generating a list of tablespaces
--innobackupex: AWS KMS plugin: loaded key 1, version 1, key length 256 bit
200326 13:05:39 [01] Streaming ibdata1
--innobackupex: Error writing file 'UNKNOWN' (Errcode: 32 "Broken pipe")
xb_stream_write_data() failed.

With the option removed SST works normally.