I'm trying to bind Header_1 to some docx style.

This sample Lua filter function should output a header level each time it finds one:

function Header(el)
  print(el.level)
end

The source reStructured text code:

***************
Header 1 sample
***************

Text

Header 2 sample
===============

Text2


Header 2 sample
===============

Text2

In a command line this should print 1, 2, 2:

pandoc --lua-filter=rst2docxHead.lua -f rst -t json test.rst

1
2
2
{"blocks":[{"t":"Header","c":[1,["header-1-sample",[],[]

So, let's see how in converts to docx…

$ pandoc --lua-filter=rst2docxHead.lua -f rst -t docx -o test.docx test.rst
1
1 

For some reason in this configuration the top level header is not processed and seems to be directly passed to docx output as a document title.