In my default.html.twig template I called them using:
{% set tutorials = page.collection('tutorials') %}
{% set blog = page.collection('blog') %}
...
{% for p in tutorials %} code
{% endfor%}
{% for p in blog %}
{% endfor%}
Blog posts from root directory only are showing up, as well as all info related! @self.children and twig commands are working.
The only problem category is not filtered and I have two identical sections showing all posts.
Rewriting should not be necessary, but I can reproduce the issue when using your excact format of frontmatter. The size of the collections were 0 when removing - '@self.children'
The reason is a Yaml indentation issue, namely the indentation of category: [excel] and category: [blog]. After correcting the indentation, the results were as expected.