There are disadvantages and advantages to both I guess, if it were done in that section your note would look a bit neater but I think it would also remove a bit of the link between a note and it's front matter. Putting it in the note itself makes it more clear what attributes you're tying to the note are but it's a bit less clean looking...

Would a middle ground be possible where if there is a front matter it gets extracted out and put in that section but if not then users can specify it when adding the article like you suggest?