Sitemap `<lastmod>` is omitted with experimental_vcs: 'default-v2' in Docusaurus 3.10.2

[eMUQI]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

After enabling:

future: {
  experimental_vcs: 'default-v2',
},

the production build completes successfully, but the generated sitemap.xml does not contain a <lastmod> value for a committed documentation page.

Using the same repository and configuration with experimental_vcs: 'git-ad-hoc' produces a <lastmod> value as expected.

Reproducible demo

https://github.com/eMUQI/docusaurus-vcs-sitemap-repro

Steps to reproduce

Clone the repository so that the Git history is available:

git clone https://github.com/eMUQI/docusaurus-vcs-sitemap-repro.git
cd docusaurus-vcs-sitemap-repro
npm ci
npm run verify:repro

The verification script performs two production builds:

1. experimental_vcs: 'default-v2'
2. experimental_vcs: 'git-ad-hoc'

It then counts the <url> and <lastmod> elements in each generated sitemap.

Expected behavior

When the sitemap plugin is configured with:

sitemap: {
  lastmod: 'date',
},

the default-v2 production build should include a <lastmod> value for the committed documentation page, as the git-ad-hoc build does.

Actual behavior

With default-v2, the generated sitemap contains the URL but no <lastmod>

Your environment

• Public source code: https://github.com/eMUQI/docusaurus-vcs-sitemap-repro
• Public site URL:
• Docusaurus version used: 3.10.2
• Environment name and version (e.g. Chrome 89, Node.js 16.4):
• Operating system and version (e.g. Ubuntu 20.04.2 LTS):

Self-service

• I'd be willing to fix this bug myself.

[eMUQI]

Additional test result: I also tested experimental_vcs: 'default-v1' with the same reproduction repository and environment. The production build completed successfully, and the generated sitemap contained one <url> and one <lastmod> entry.

experimental_vcs	<url> count	<lastmod> count
default-v1	1	1
default-v2	1	0
git-ad-hoc	1	1

This confirms that default-v1 produces the expected <lastmod> value in this reproduction.