This happened to be when upgrading from 10.3 to 10.4, but I suspect other versions are affected. I use sysv-rc to launch daemons/servers . I was forced to install systemd for the sake of php which makes use of systemd-tmpfiles command. But I otherwise do not use systemd at all. I think since this got installed, mariadb's installer assumed I use systemd for init. Therefore it failed to add the mysql.init script to /etc/init.d/ . This means mysql won't start now. I was able to manually yank mysql.init from the source and that works fine, but I hate to have to manually do this everytime I update (what if you guys make changes to the init script for example).
I think your installer script needs to be a little more thorough about checking that systemd is being used for init.