The mariadb 10.4 server fails to start using command
/usr/local/libexec/mysqld --user=root --defaults-file=/usr/local/etc/my.cnf
2019-08-17 15:36:56 0 [ERROR] /usr/local/libexec/mysqld: unknown variable 'defaults-file=/usr/local/etc/my.cnf'
It looks like a failure to correctly parse the command line, with the result that it interprets both the option and its value as a single, unrecognisable variable.
I forgot to select "affects many"