Indeed, this look completely wrong.

This might be a duplicate of #9866.

Similar difference in ->y value was introduced with PHP 8.1.10
https://3v4l.org/Wiane
It returns -1 instead of 0 when 2 dates with different timezones are compared.

<?php
$utc = new DateTimeZone('UTC');
$london = new DateTimeZone('Europe/London');

$test1 = new \DateTime('2018-01-31', $utc);
$test2 = new \DateTime('2018-03-31 00:00:00', $london);

$dateDiff = $test2->diff($test1); // true absolute Should the interval be forced to be positive
//$dateDiff = $test1->diff($test2);
$bugs = array('y' => &$dateDiff->y, 'm' => &$dateDiff->m, 'd' => &$dateDiff->d, 'h' => &$dateDiff->h, 'invert' => &$dateDiff->invert, 'days' => &$dateDiff->days);
$test3 = new \DateTime('+2 month ' . $test1->format('Y-m-d\\TH:i:s\\Z'));

var_dump($test3->format('Y-m-d H:i:s +00:00'), $bugs);

?>

php >= 8.1.10

string(26) "2018-03-31 00:00:00 +00:00"
array(6) {
  ["y"]=>
  int(-1)
  ["m"]=>
  int(10)
  ["d"]=>
  int(0)
  ["h"]=>
  int(1)
  ["invert"]=>
  int(0)
  ["days"]=>
  int(58)
}

php all
If set $test2 = new \DateTime('2018-03-31 01:00:00', $london); and $test2->diff($test1) it's partial correct with inverter 1, if $test1->diff($test2) is correct

php < 8.1.10

string(26) "2018-03-31 00:00:00 +00:00"
array(6) {
  ["y"]=>
  int(0)
  ["m"]=>
  int(1)
  ["d"]=>
  int(30)
  ["h"]=>
  int(23)
  ["invert"]=>
  int(1)
  ["days"]=>
  int(58)
}

if uncomment $dateDiff php < 8.1.10

string(26) "2018-03-31 00:00:00 +00:00"
array(6) {
  ["y"]=>
  int(0)
  ["m"]=>
  int(1)
  ["d"]=>
  int(27)
  ["h"]=>
  int(23)
  ["invert"]=>
  int(0)
  ["days"]=>
  int(58)
}

@cmb69 it's great bug exists for function diff (don't only duplicate)

I am for restoring before creating same type timezone of php 8.1.10, adding a note in the manual for this use case otherwise the code needs improvement and I mean if different from same type it uses the same hacks that would be used with timezone type 3 Europe/Rome == Europe/Rome ... it is difficult to rewrite same type for this type of use.

Now fixed, for PHP 8.1.14 and PHP 8.2.1.