Thanks!
I have found the issue, it is unfortunately the theme. In the layout code, it uses a custom loop, which only allows posts, featured_item and page post types, please see screenshot of the related code here: https://i.imgur.com/KHuKX7R.png
If I change it to any and remove the last code block, then it works: https://i.imgur.com/rRZCItG.png
However making this change is a really bad idea, as any future update to the theme will override it.
There is probably a reason behind this, the grid might be designed for only those post types. You can let the theme author know, this might be just a bug. If it’s intentional, then he could add a hook to change these query args in a future release.