Sitemaps fail to update with images inside a slideshow block

### Impacted plugin

Jetpack

### Quick summary

@vparkhere reported this issue on her site where the sitemap won't update despite adding images to a post, specifically via a slideshow block. She gave the sitemap 12 hours to update on its own, but it still didn't update properly with the images that added to the post.

Here's the test post with the slideshow: https://vpat001.wordpress.com/2017/04/19/first-blog-post/
Test site's sitemap: https://vpat001.wordpress.com/sitemap.xml

### Steps to reproduce

1. Make sure site is public, then add a slideshow block with images to either a new post or published post. Publish/Update.
2. Check site map at URL/sitemap.xml
3. Wait for a while, at least 10 minutes, and check site map again. Still will not show images that were added to the post despite lastmod date being updated to latest publish/update date.

### A clear and concise description of what you expected to happen.

Images added to the post to appear in the sitemap

### What actually happened

Images were not updated in the sitemap.

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Screenshot of sitemap with no images for the post https://vpat001.wordpress.com/2017/04/19/first-blog-post/:

### Impact

One

### Available workarounds?

There is no user impact

### Platform (Simple and/or Atomic)

Simple

### Logs or notes

Originally reported here: https://github.com/Automattic/wp-calypso/issues/80973#issuecomment-1718048615