The conf directory is different between mysql and mariadb.

defaults/main.yml

@@ -3,7 +3,7 @@ mysql_enabled: true
 mysql_installs_mariadb: true
 mysql_pkg_state: present
 mysql_conf_dir: /etc/mysql/conf.d
-mysql_service_conf_dir: /etc/mysql/mysql.conf.d
+mysql_service_conf_dir: "{% if mysql_installs_mariadb %}/etc/mysql/mariadb.conf.d{% else %}/etc/mysql/mysql.conf.d{% endif %}"
 mysql_socket: /run/mysqld/mysqld.sock
 mysql_data_dir: /var/lib/mysql
 mysql_log_dir: /var/log/mysql