Intl::composeLocale does not consider subtags

Description

The following code:

<?php

$first = 'en_US@timezone=Europe/Amsterdam';
$second = 'nl_NL';

var_dump(\Locale::parseLocale($first));
var_dump(\Locale::getKeywords($first));
var_dump(\Locale::parseLocale($second));
var_dump(\Locale::getKeywords($second));

$merged = [
    ...\Locale::parseLocale($first),
    ...\Locale::getKeywords($first) ?? [],
    ...\Locale::parseLocale($second),
    ...\Locale::getKeywords($second) ?? [],
];

var_dump($merged);

var_dump(\Locale::composeLocale($merged));

$newSynax = 'en_US-u-tz-nlams';

var_dump(\Locale::parseLocale($newSynax));
var_dump(\Locale::getKeywords($newSynax));

Resulted in this output:

array(2) {
  ["language"]=>
  string(2) "en"
  ["region"]=>
  string(2) "US"
}
array(1) {
  ["timezone"]=>
  string(16) "Europe/Amsterdam"
}
array(2) {
  ["language"]=>
  string(2) "nl"
  ["region"]=>
  string(2) "NL"
}
NULL
array(3) {
  ["language"]=>
  string(2) "nl"
  ["region"]=>
  string(2) "NL"
  ["timezone"]=>
  string(16) "Europe/Amsterdam"
}
string(5) "nl_NL"
array(2) {
  ["language"]=>
  string(2) "en"
  ["region"]=>
  string(2) "US"
}
NULL

But I expected this output instead:

array(2) {
  ["language"]=>
  string(2) "en"
  ["region"]=>
  string(2) "US"
}
array(1) {
  ["timezone"]=>
  string(16) "Europe/Amsterdam"
}
array(2) {
  ["language"]=>
  string(2) "nl"
  ["region"]=>
  string(2) "NL"
}
NULL
array(3) {
  ["language"]=>
  string(2) "nl"
  ["region"]=>
  string(2) "NL"
  ["timezone"]=>
  string(16) "Europe/Amsterdam"
}
string(5) "nl_NL@timezone=Europe/Amsterdam"
array(2) {
  ["language"]=>
  string(2) "en"
  ["region"]=>
  string(2) "US"
}
array(1) {
  ["timezone"]=>
  string(16) "Europe/Amsterdam"
}

The reasoning behind this, as the example shows, is that I try to activate a new language and region while maintaining the other subtags.

PHP Version

PHP 8.2-8.5 and master

Operating System

No response