Sorry, stupid of me not to check for multiple site-map files.
Everything seems in order!

Thanks about the cloud back-end tip!

every time you publish a blog article, this regenerate your sitemaps!
you can check your robots.txt file (also generated on publish)
eg: https://appdrag.com/robots.txt

there you can see the sitemaps are declared:

User-agent: *
Sitemap: https://appdrag.com/sitemap.xml
Sitemap: https://appdrag.com/sitemap-blog.xml

and then you can check the specific sitemap for blog:
eg: https://appdrag.com/sitemap-blog.xml

So cloudbackend is not required here, but let say you have content in your database that you want to expose, you can create a cloud function to generate a sitemap for you, we have a ready to use sample in node.js available

I really need a solution to this, since blog is a big part of the current project we're working on, it just HAS to be indexed.

And don't tell me we have to use back-end for this too... that would be the last straw.

That's what I expected, so I re-generated it and waited 10 hours and they are not there.

Also, only relying on the site-map is bad, because then we're expected to submit it manually to all known and unknown search engines.