To start multiple instances of MariaDB, you added a /lib/systemd/system/mariadb@.service, indicating that we must create config file for the second daemon in /etc/mysql/conf.d/my{instancename}.cnf.

But this file is read by the main daemon! In /etc/mysql/my.cnf, we found at the bottom:
!includedir /etc/mysql/conf.d/*

So the main daemon:

• reads the port/socket/etc. from my.cnf
• change those values later when reading my{instancename}.cnf

I you comment the !includedir and add as many !include /etc/mysql/conf.d/{conf}.cnf you have, it works as expected.

To my mind, the "clean" way of fixing it si to read those my{instancename}.cnf files from /etc/mysql, and NOT from /etc/mysql/conf.d.