Here is the bigger picture behind all this:
sitemap_recent.xml:
- Only includes all the changed topics from the last 3 days
- Is renewed every 1 h (Internal Rails cache time of 1 h)
- Has correct lastmod date in
sitemap.xml
sitemap_[1-5].xml:
- Really includes all and every topic, and also includes all the changed topics from the last 3 days
- Is renewed every 24 h (Internal Rails cache time of 24 h)
sitemap_[2-5].xmlhave wrong lastmod date of3.days.agoinsitemap.xml
The wrong lastmod date for sitemap_[2-5].xml does not matter, as Google will get all recent topic changes via sitemap_recent.xml in a timely manner.