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I take it this advice about custom frontmatter is now outdated however as tags now seem to be officially supported and custom frontmatter in the top-most YAML block seems to be widely used?