After make install clean && rehash for databases/mariadb105-server (10.5.8) and databases/mariadb105-client, I ran:
sysrc mysql_enable="YES" (no issues)
service mysql-server start
chown: mysql: illegal user name
Cannot change ownership of the database directories to the 'mysql' user. Check that you have the necessary permissions and try again.
/user/local/etc/rc.d/mysql-server: WARNING: failed precmd routine for mysql
There was a mismatch in user accounts somewhere. It was a fresh install of FreeBSD 12.2, upgraded from 12.1. I may be mistaken but perhaps that upgrade process tripped up the user accounts.
In the end to resolve this issue, I ran:
Then removed the last line, which was a "mysql" user.
I also trashed the contents of /var/db/mysql.