I happened to notice that the issue you created has been closed. There is a comment by @jmooring saying my layout directory structure is unexpected.
I have two questions, but before I ask them, I will explain what I tried to achieve with this template structure (which worked fine before the update).
I want to have a separate landing page for a blog (example.com/blog) that looks different than a single post (example.com/2025/my-post/) or a yearly archive list (https://example.com/2025/). The root of the blog is not just a simple list view. That is why I have three different templates, and that is also why index.md contains a separate layout definition in the front matter.
So, my questions are:
Why does this work when the layout is called root but not when it is called index? Based on the documentation, the layout field in the front matter “provide a template name to target a specific template, overriding the default template lookup order.”
What would be the less unexpected or the correct way to achieve the same end result?