It seems that detecting lengths on strings will use the strlen() function. This causes issues when detecting multibyte strings, as strlen() count bytes, not characters.
Example:
$v->required('user.first_name')->lengthBetween(2, 5)->alpha();
$result = $v->validate([
'user' => [
'first_name' => 'Björk',
],
]);
will thrown an error on validation, indicating that the length is too long (although bytewise correct, it's doubtful that this would be the expected behaviour)