Description:
------------
after adding 1 microsecond to datetime with 999999 microseconds, resulted datetime has the same seconds and 1000000 microseconds :]

see the second dump
https://3v4l.org/ZbGpl

Test script:
---------------
<?php

$d = new \DateTimeImmutable('2000-01-01 23:59:59.999999');
$d2 = $d->modify('+1 microsecond');
var_dump($d);
var_dump($d->format('s.u'));
var_dump($d2);
var_dump($d2->format('s.u'));