Nested data iterations

Is there a reason why nested data iterations should not work?
The following will output the county files, but ignores counties_with_articles. Seems like it ignores the inner iterations completely?

data.counties.each do |county|
  proxy (...)
  data.counties_with_articles do |county_with_articles|
     county_with_articles.articles.each do |article|
        proxy (using county.name, county.slug etc..)
     end
  end
end

We ran into the same issue after we upgraded to MM4.3.7
The way we finally cracked it was to call @app.data.counties.each and @app.data.counties_with_articles
Try that and see if it helps.