.mentions is empty

Here's an ancient problem that I'd like to see cleared up, in order to make progress on new features for Project + Reviews plugin.

The plugin depends on reading metadata in notes, for example
#area @start(2021-09-29) @review(6w) @reviewed(2023-06-23)

To do this I first try to get the list of all mentions:

let mentions: $ReadOnlyArray<string> = note.mentions ?? []

but normally it finds none, despite there being many in the note.

So I then have to use this fallback:

const metadataLineIndex = getOrMakeMetadataLine(note)
let metadataLine = paras[metadataLineIndex].content
mentions = (metadataLine + ' ').split(' ').filter((f) => f[0] === '@')

I've put a warning in the plugin-side docs that this call is unreliable.