mariadb parallel slave server shutdown hang - v10.4.8

During on-site engagement, I installed master-slave configuration with MaxScale v2.3 on top of master node.

OS: CentOS 7.4.1708
MariaDB: v10.4.8
MaxScale: v2.3.11

Customer is using GTID to utilize auto failover of MaxScale and HA test has been done successfully.

But when I tried to shutdown mariadb server it failed with timeout.
I tried
$ systemctl stop mysql
and
$ mysqladmin shutdown and the result is the same.

The last error log is follows..
...
[Note] Slave I/O thread: connected to master 'xxx@xxxx:3306', replication starts at GTID position '0-1000-382'
----------------------------------------------

So If I stop the slave with "mysql> stop slave;" and then tried to stop the mariadb server, it stops immediately.
It looks like shutdown never stops "Slave I/O thread"... and got timeout.

Is it a known issue of v10.4.8 or any OS setting dependancies ?

FYI. innodb-thread-concurrency is set to 0.

Thanks