Thanks a lot for your help, which enabled me to write the following short 
filter, which puts the name of all authors on the same line in 'docx' 
output, while preserving the nice default layout in 'latex' output.
Debugging filters working on the yaml metadata provided a particular 
challenge to me:

function Meta (m)
  if FORMAT:match 'docx' then
        authors = m.author[1] .. {pandoc.Str(', ')} .. m.author[2]
        m.author = pandoc.MetaInlines(authors)
  end
  return m
end

It only handles two and not more authors, but in my case, that's fine.
William Lupton schrieb am Mittwoch, 14. Juni 2023 um 18:55:04 UTC+2:

> 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 <stephan2...-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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