The following Lua (used with RMarkdown in RStudio) filter should put "Figure n." in bold at the beginning of every figure caption, but it doesn't change my output:

function Image (img)
if FORMAT:match 'docx' then
caption = pandoc.utils.stringify(img.caption)
if (string.find(caption, 'Fig') ~= nil) then
img.caption[1] = pandoc.Strong(img.caption[1])
img.caption[3] = pandoc.Str(string.gsub(img.caption[3].text, ":", "."))
img.caption[3] = pandoc.Strong(img.caption[3])
fig_num_string = string.sub(pandoc.utils.stringify(img.caption[3]),1,2)
fig_num = math.floor(tonumber(fig_num_string))
if (fig_num > 6) then
img.caption[3] = pandoc.Strong("S" .. tostring(8-fig_num) .. '.')
end
img.caption.long = pandoc.Strong('A')
img.caption = pandoc.Strong('A')
end
end
print(pandoc.utils.stringify(img.caption.long))
return img
end

By putting print statements, I can partially verify that the filter operates on the correct elements, but it doesn't change the output.