If I change the number of related posts on my own site, using a different theme, to show six, it shows them in rows of two, without breaks like you’re seeing on your site. If I change it to nine, same. In fact, on my site on a post that shows nine on desktop, it only shows 8 on mobile, so as to not show an empty space next to the last one.
See: https://photos.kokkieh.blog/2018/11/01/hunter/
If we’re using the same version of Jetpack, and we’re using the same code snippet from https://jetpack.com/support/related-posts/customize-related-posts/#number to customize this, the only other factor that could be influencing things is the theme.
I looked at your related posts in the browser inspector, and it appears the code causing this is coming from several rules starting at line 5112 in the 303dd.css
file on your site. That file is not added by Jetpack, so it’s either coming from your theme or from another plugin.