> But why is it that I cannot make Pandoc use a specific identifier like in
> 
> $ printf '# 47 {#47}\n' | pandoc 
> <h1 id="section">47 {#47}</h1>
> 
> I expected Pandoc to produce `id="47"`.

As a workaround you can write '# 123 {id=123}\n' but not being able to
use the #-syntax seems like a bug to me.