What do you mean 'you update'?
Plesk doesn't ship MariaDB, it only downloads it from original OS vendor repo and it is quite likely it may be outdated there (especially for older OS). If your OS comes with MariaDB 10.1.21, then Plesk would run MariaDB 10.1.21 for clients.
[root@ppu17-0 templates]# rpm -qi mariadb | grep Vendor
Vendor : CentOS