Thanks @scholarInTraining that helps to clarify. It would probably be worth mentioning in the docs. I had a look through but couldn’t see any mention of it when it’s actually quite an important thing to note before you travel too far down the wrong path.
I take it this advice about custom frontmatter is now outdated however as tags now seem to be officially supported and custom frontmatter in the top-most YAML block seems to be widely used?
Yes both those things are true. I agree that the Obsidian help page on frontmatter could use some additional information about formatting and the other details you mention. I think there’s a way to suggest changes for those somewhere, perhaps on GitHub? Maybe others know more.