Lets try and set check_implicit_admin true here to see if that allows it to set the password correctly.

roles/mariadb_server/tasks/main.yml

@@ -22,6 +22,7 @@
 
 - name: set mariaddb root user password
   no_log: True
+  check_implicit_admin: True
   mysql_user: name=root password={{ mariadb_root_password }}
 
 - name: create .my.cnf file for future logins