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.slug etc..)

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