Platform mu-plugin always 301s /sitemap.xml and blocks third-party sitemaps #4325

Prerequisites

• I have carried out troubleshooting steps and I believe I have found a bug.
• I have searched for similar bugs in both open and closed issues and cannot find a duplicate.

Describe the bug

writeCommonPlatformMuPlugins() writes /internal/shared/mu-plugins/sitemap-redirect.php on every boot:

https://github.com/WordPress/wordpress-playground/blob/trunk/packages/playground/wordpress/src/platform-mu-plugins.ts

That file runs at muplugins_loaded and header() + exits any {site path}/sitemap.xml request (including ?… and /…) to {site path}/wp-sitemap.xml with a 301. WordPress rewrite matching and regular plugins never see the URL. There is no filter and no opt-out.

The comment says this is for “non-root installations” because Core only adds a sitemap.xml → wp-sitemap.xml rewrite at the domain root, and cites #2051.

That comment is wrong on three points:

1. After core#61931 (6.7), Core does not 301 /sitemap.xml to /wp-sitemap.xml. WP_Sitemaps::redirect_sitemapxml() is deprecated. Core registers an internal rewrite in WP_Rewrite::rewrite_rules():

   sitemap.xml → index.php?sitemap=index

   Same query as wp-sitemap.xml (WP_Sitemaps::register_rewrites()). The sitemap is served at /sitemap.xml. The old redirect was only for when /sitemap.xml was parsed as a page named sitemap-xml.

2. “Installed at the root” in that Core block is home_url() path empty or / — the same guard as robots.txt and favicon.ico. A subdomain (blog.example.com/) is root. A subdirectory (example.com/blog/) is not. There is no Trac ticket asking Core or Playground to 301 /sitemap.xml for subdirectory or subdomain installs. A subdomain is not a Playground-only edge case.

3. playground.wordpress.net and CLI http://127.0.0.1:PORT/ are root. sitemap.xml does not render for WordPress 6.7 in Playground web app #2051 / [Website] Fix sitemap.xml redirect to wp-sitemap.xml  #3140 already flush rewrite rules so Core’s rule exists. This mu-plugin still 301s those sites.

Playground should not special-case this URL. Any WordPress site can run on a subdomain; that is Core's root case.

Expected behavior

Playground does not intercept /sitemap.xml.

• Vanilla, pretty permalinks, site at /: Core's rewrite may serve the Core sitemap at /sitemap.xml (200). No platform 301.
• A sitemap plugin that owns /sitemap.xml can respond. Playground does not steal the request first.

Actual behavior

Every {site path}/sitemap.xml request gets:

Status: 301 Location: {site path}/wp-sitemap.xml

On a vanilla site this looks “correct” because /wp-sitemap.xml then serves Core's sitemap. It is still not what Core does after 6.7, and it hides the hijack.

With a plugin that serves /sitemap.xml (e.g., The SEO Framework), the plugin sitemap is never returned.

Steps to reproduce

Vanilla (no extra plugins):

1. Open https://playground.wordpress.net/?url=%2Fsitemap.xml (or @wp-playground/cli server, then request /sitemap.xml without following redirects).
2. See 301 Location: /wp-sitemap.xml.
3. On a normal root WordPress 6.7+ install with pretty permalinks, the same URL is an internal rewrite, not this 301.

With a sitemap plugin:

1. Same vanilla site, default theme.
2. Activate only a plugin that serves /sitemap.xml (example: The SEO Framework).
3. Open /sitemap.xml.
4. See a 301 to /wp-sitemap.xml instead of the plugin's sitemap XML.

Isolating the problem

• I have deactivated other plugins and confirmed this bug occurs when only this plugin is active.
• This bug happens with a default WordPress theme active.
• I can reproduce this bug consistently using the steps above.