The bug:
Line 39:
echo mysql2date('Y-m-d\TH:i:s +0000', get_post_time('Y-m-d H:i:s', true), false);
This causes the lastmod to be the post publish time. Which is always unchanged and make the search engine to not crawl the post.
Here is the correct code:
echo mysql2date('Y-m-d\TH:i:s +0000', get_post_modified_time('Y-m-d H:i:s', true), false);
My request: Please fix this in the plugin code for future releases.
Reference: I found this bug when checking the site map for Cheap Cars for Sale: cars.apmart.com/sitemap.xml