I already mentioned this topic before and pointed out that this is not an optimal solution.
It is admittedly a bit cumbersome and involves more manual work each time, but you’ll need to manually remove the checkmark for “Include in sitemap” in your .md files.
BTW: It’s also known that the blog URLs are currently not being indexed by Google. This is because the CMS sets the same canonical URL for all blog posts, so every post points to the canonical URL of the main page.
I’ve already raised this issue before. Due to the many things that still need to be addressed, the CMS is likely still in beta and not yet fully functional.