Hi,

Could you please describe you galera setup, MySQL and galera versions, and how many nodes and configuration of the nodes. I could not repeat your problem using 2-node galera cluster as seen below:

jan@jan-GE70-0NC-0ND ~/mysql/galera-5.5 $ /usr/local/mysql/bin/mysql -u root -S /home/jan/mysql/galera-cluster-5.5/node0/mysql.4000.sock
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 6
Server version: 5.5.34-MariaDB-wsrep-debug-log Source distribution, wsrep_23.7.6.r3457

Copyright (c) 2000, 2013, Oracle, Monty Program Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
MariaDB [test]> CREATE TABLE `user` (
    ->   `id` int(11) NOT NULL AUTO_INCREMENT,
    ->   `userid` varchar(30) DEFAULT NULL,
    ->   `passwd` varchar(30) DEFAULT NULL,
    ->   `domain` varchar(30) DEFAULT NULL,
    ->   `homeserver` varchar(30) DEFAULT NULL,
    ->   `quotaInMiB` int(11) DEFAULT '100',
    ->   `password_type` varchar(10) DEFAULT 'plain',
    ->   `spamaction` enum('REJECT','ACCEPT','TAGGING') DEFAULT 'REJECT',
    ->   `spampoint` decimal(5,1) DEFAULT '5.2',
    ->   `virusaction` enum('REJECT','ACCEPT','REPLACE') DEFAULT 'REJECT',
    ->   `keepFwdCopy` tinyint(1) DEFAULT '0',
    ->   `quotaUsed` int(11) DEFAULT '0',
    ->   `extRef` varchar(12) DEFAULT NULL,
    ->   `mandate` int(11) DEFAULT '0',
    ->   `FwdBounceCount` decimal(1,0) DEFAULT '0',
    ->   `FwdActive` tinyint(1) DEFAULT '1',
    ->   `lastActive` datetime DEFAULT '0000-00-00 00:00:00',
    ->   `lastLogin` datetime DEFAULT '0000-00-00 00:00:00',
    ->   `disabled` tinyint(1) DEFAULT '0',
    ->   PRIMARY KEY (`id`),
    ->   UNIQUE KEY `userid` (`userid`,`domain`)
    -> ) ENGINE=InnoDB AUTO_INCREMENT=26944 DEFAULT CHARSET=utf8;
Query OK, 0 rows affected (0.20 sec)

MariaDB [test]>
MariaDB [test]> insert into user(id, userid, domain) values (1, 'jan','test');
Query OK, 1 row affected (0.00 sec)

MariaDB [test]> insert into user(id, userid, domain) values (5, 'benoit','ambonet.ch');
Query OK, 1 row affected (0.01 sec)

MariaDB [test]> insert into user(id, userid, domain) values (7, 'kerr','test');
Query OK, 1 row affected (0.00 sec)

MariaDB [test]> commit work;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> UPDATE user set lastLogin='2013-12-19 14:55:09' where
    -> concat(userid,'@',domain)='benoit@ambonet.ch';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

MariaDB [test]>
MariaDB [test]> UPDATE user set lastLogin='2013-12-19 14:55:16' where
    -> concat(userid,'@',domain)='benoit@ambonet.ch';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

MariaDB [test]>
MariaDB [test]> UPDATE user set lastLogin='2013-12-19 14:55:17' where
    -> concat(userid,'@',domain)='benoit@ambonet.ch';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

MariaDB [test]>
MariaDB [test]> UPDATE user set lastLogin='2013-12-19 14:55:17' where
    -> concat(userid,'@',domain)='benoit@ambonet.ch';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

MariaDB [test]>
MariaDB [test]> UPDATE user set lastLogin='2013-12-19 14:55:17' where
    -> concat(userid,'@',domain)='benoit@ambonet.ch';
Query OK, 0 rows affected (0.01 sec)
Rows matched: 1  Changed: 0  Warnings: 0