CakePHP 3: Collection: listNested - cakephp

Question

I want to flatten a tree structure with CakePHP's listNested function as explained here: http://book.cakephp.org/3.0/en/core-libraries/collections.html#Cake\Collection\Collection::listNested
I've tried the given example, but I don't get the expected result as in the example. What am I doing wrong or is this a bug in the listNested function?
My code:
$data = [
[
'id' => 1,
'parent_id' => null,
'name' => 'Birds',
'children' => [
['id' => 2, 'parent_id' => 1, 'name' => 'Land Birds', 'children' => []],
['id' => 3, 'parent_id' => 1, 'name' => 'Eagle', 'children' => []],
['id' => 4, 'parent_id' => 1, 'name' => 'Seagull', 'children' => []],
]
],
[
'id' => 6,
'parent_id' => null,
'name' => 'Fish',
'children' => [
['id' => 5, 'parent_id' => 6, 'name' => 'Clown Fish', 'children' => []],
]
]
];
$collection = new Collection($data);
$flatList = $collection->listNested();
debug($flatList->toArray());
My result:
[
(int) 0 => [
'id' => (int) 5,
'parent_id' => (int) 6,
'name' => 'Clown Fish',
'children' => []
],
(int) 1 => [
'id' => (int) 6,
'parent_id' => null,
'name' => 'Fish',
'children' => [
(int) 0 => [
'id' => (int) 5,
'parent_id' => (int) 6,
'name' => 'Clown Fish',
'children' => []
]
]
],
(int) 2 => [
'id' => (int) 4,
'parent_id' => (int) 1,
'name' => 'Seagull',
'children' => []
]
]
Expected result:
[
['id' => 1, 'parent_id' => null, 'name' => 'Birds'],
['id' => 2, 'parent_id' => 1, 'name' => 'Land Birds'],
['id' => 3, 'parent_id' => 1, 'name' => 'Eagle'],
['id' => 4, 'parent_id' => 1, 'name' => 'Seagull'],
['id' => 6, 'parent_id' => null, 'name' => 'Fish'],
['id' => 5, 'parent_id' => 6, 'name' => 'Clown Fish']
]