Hello @bohnonlos,
Thanks for using the Yoast SEO plugin.
Checking this post, I noticed that meta tags such as article:published_time and datePublished are missing from the Yoast SEO meta tags and structured data output.
Instead, you have <meta itemprop="datePublished" content="1970-01-01T00:00:00+00:00"> . It could be that another plugin is responsible for that schema output and conflicting Yoast SEO’s output, thereby affecting the lastmod in the sitemap. So you may have not modified the article dates manually, but another plugin might be responsible with tampering with it.
Often, we see problems occur in combination with another plugin or theme. The fastest way to rule out any conflict is to deactivate all non-Yoast plugins and switch to a standard theme like Twenty Twenty-Two.
Please test this on your development or staging site if you have one. If not, we recommend using the Health Check & Troubleshooting plugin. This plugin has a troubleshooting mode, which does not affect normal visitors to your site.
If you’re unfamiliar with checking for conflicts, we’d like to point you to a step-by-step guide that will walk you through the process: How to check for plugin conflicts
If you feel uncomfortable doing this yourself or if this does not solve your issue, our Yoast SEO Premium plugin comes with one year of (technical) support.
