Mysql – auto_increment incremented on non-affecting queries

When performing the following query on 'sessions':

INSERT IGNORE INTO sessions (user_id, archived) VALUES (3, 0)

while:

CREATE TABLE `sessions` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `user_id` bigint(20) unsigned NOT NULL,
  `date_created` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `date_modified` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `archived` tinyint(3) unsigned NOT NULL,
  `valid_until` datetime NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id_UNIQUE` (`id`),
  UNIQUE KEY `user_id_UNIQUE` (`user_id`),
  KEY `fk_sessions_users1_idx` (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=18 DEFAULT CHARSET=utf8;

with trigger:

CREATE TRIGGER before_insert_on_sessions BEFORE INSERT ON `sessions`
FOR EACH ROW SET new.valid_until = IFNULL(new.valid_until,DATE_ADD(NOW(), INTERVAL 1 DAY));

the first time you execute the query, everything is fine and the following is inserted:

id  user_id date_created    date_modified   archived    valid_until
1   3       ...             ...             0           ...

now enter the same query two more times, and it will say:

No errors, 0 rows affected, taking 8,0ms

Now change the query to:

INSERT IGNORE INTO sessions (user_id, archived) VALUES (4, 0)

So basically you only changed the value of user_id.

You'd expect the next row to be:

id  user_id date_created    date_modified   archived    valid_until
**2**   4       ...             ...             0           ...

instead, you get:

id  user_id date_created    date_modified   archived    valid_until
**4**   4       ...             ...             0           ...

So basically what's happening, is that the unique_key (id)'s auto_increment is being updated on every query, even though there are no actual new rows being made.

This shouldn't be right, should it?

Server version:     5.6.22 Homebrew