RSS feeds read by WordPress are ordered by date by default. Use `wp_feed_options` action to disable date ordering and consume the items in their own order.

Disable WordPress RSS feed date ordering

<?php

// E.g. fetch_feed('https://www.example.com/some-feed');

add_action( 'wp_feed_options', 'my_feed_options', 10, 2 );
function my_feed_options( SimplePie $feed, $url ): void {
  $my_feed = 'https://www.example.com/some-feed';

  if ( ! is_string($url) || $my_feed !== $url ) {
  	return;
  }

  // https://simplepie.org/wiki/reference/simplepie/enable_order_by_date
  $feed->enable_order_by_date(false);
}