Unexpected results with equal many-to-many relations

INSERT INTO match_reference (match_id, user_id) VALUES (?, ?)
Array
(
    [0] => 2
    [1] => 1
)


UPDATE match_reference SET match_id = ? WHERE user_id = ? AND match_id = ?
Array
(
    [0] => 1
    [1] => 1
    [2] => 2
)


INSERT INTO match_reference (match_id, user_id) VALUES (?, ?)
Array
(
    [0] => 3
    [1] => 2
)


INSERT INTO match_reference (match_id, user_id) VALUES (?, ?)
Array
(
    [0] => 4
    [1] => 2
)
Array
(
    [0] => Array
        (
            [id] => 2
            [username] => 2
        )

)
Array
(
    [0] => Array
        (
            [id] => 1
            [username] => 1
        )

    [1] => Array
        (
            [id] => 3
            [username] => 3
        )

    [2] => Array
        (
            [id] => 4
            [username] => 4
        )

)
Array
(
)
Array
(
    [0] => Array
        (
            [id] => 3
            [username] => 3
        )

    [1] => Array
        (
            [id] => 4
            [username] => 4
        )

)

---
User:
  columns:
    username: string(255)
  relations:
    Matches:
      class: User
      local: user_id
      foreign: match_id
      refClass: MatchReference
      equal: true
      cascade: [delete]

MatchReference:
  columns:
    user_id:
      type: integer
      primary: true
    match_id:
      type: integer
      primary: true

require_once('config.php');

Doctrine::loadModels('models');

Doctrine_Query::create()
    ->delete('User')
    ->execute();

Doctrine_Query::create()
    ->delete('MatchReference')
    ->execute();

$u1 = new User();
$u1->id = $u1->username = 1;
$u1->save();

$u2 = new User();
$u2->id = $u2->username = 2;
$u2->save();

$u3 = new User();
$u3->id = $u3->username = 3;
$u3->save();

$u4 = new User();
$u4->id = $u4->username = 4;
$u4->save();

$profiler = new Doctrine_Connection_Profiler();
Doctrine_Manager::getInstance()->getCurrentConnection()->addListener($profiler);

$user_1 = Doctrine::getTable('User')->find(1);

$matches = Doctrine_Query::create()
    ->from('User')
    ->whereIn('id', array(2))
    ->execute(array(), Doctrine::HYDRATE_ARRAY);

$match = $matches[0];

$user_1->link('Matches', array($match['id']));
$user_1->save();

$user_2 = Doctrine::getTable('User')->find(2);

$matches = Doctrine_Query::create()
->from('User')
->whereIn('id', array(3, 4))
->execute(array(), Doctrine::HYDRATE_ARRAY);

foreach ($matches as $match) {
    $user_2->link('Matches', array($match['id']));
}

$user_2->save();

foreach ($profiler as $event) {
    if (in_array($event->getName(), array('execute')) && preg_match('/^(INSERT|UPDATE)/', $event->getQuery())) {
        echo print_r($event->getQuery(), true) . "\n";
        echo print_r($event->getParams(), true) . "\n\n";
    }
}

print_r($user_1->Matches->toArray());
print_r($user_2->Matches->toArray());

$user_1->refresh(true);
$user_2->refresh(true);

print_r($user_1->Matches->toArray());
print_r($user_2->Matches->toArray());