Your metadata has this structure (hmm... perhaps some of the Inlines are
really MetaInlines, in which case that's a problem with my logging module):

(#) meta Meta {
  author: List[2] {
    [1] Inlines[3] {
      [1] Str "Author"
      [2] Space
      [3] Str "A"
    }
    [2] Inlines[3] {
      [1] Str "Author"
      [2] Space
      [3] Str "B"
    }
  }
  title: Inlines[1] {
    [1] Str "Test"
  }
}

... so I think you want this filter (MetaInlines isn't an attribute name,
and you have to count the space):

function Meta (m)
    print(pandoc.utils.stringify(m.author[1][3]))
end

On Wed, 14 Jun 2023 at 17:47, Stephan Boltzmann <stephan2boltzmann-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:

> Hello everybody out there using Pandoc,
>
> For the following short document,
> ---
> title: Test
> author:
>   - Author A
>   - Author B
> ---
> # Introduction
>
> I would expect the Lua filter
>
> function Meta (m)
>     print(pandoc.utils.stringify(m.author[1].MetaInlines[2]))
> end
>
> to print out "A", but it prints nothing at all.
>
> What is the conceptual misunderstanding I have here?
>
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>

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