Table->find() does not reset the modified field list

// Create some object in the database.
$instance = new Model();
$instance->field = "value 1";
$instance->save();
$id = $instance->id;

$table = Doctrine::getTable('Model');

// Load the object from the database.
$instance_a = $table->find($id);

// Modify a field. 
$instance_a->field = "value 2";
print $instance_a->field . "\n";     // prints "value 2";
print_r($instance_a->getModified()); // prints "field"

// Load the object again from the database, into a new variable.
$instance_b = $table->find($id);

// $instance_a and $instance_b now are the same object.
// The data is fresh data from the database.
print $instance_a->field . "\n"; // prints "value 1";
print $instance_b->field . "\n"; // prints "value 1";

// However, the modified field list was not reset.
print_r($instance_a->getModified()); // prints "field"
print_r($instance_b->getModified()); // prints "field"