ActiveAdmin bug? Nested form in show resource page does not display form submit button

I am using ActiveAdmin to build an article show page where new comments can be added using a nested form but facing a problem. Article has_many :comments and also accepts_nested_attributes_for :comments. It all works fine in Rails console but I can't get the form submit button to show up. Here is my code for customizing activeadmin's show page for Article:

show do |article|
  attributes_table do
    row :id
    row :title 
  end

  div do
    semantic_form_for [:admin, article], builder: ActiveAdmin::FormBuilder do |f|
      f.has_many :comments, allow_destroy: false, new_record: true, heading: false do |g|
        g.input :body
      end
      f.actions
    end
  end
end

The nested form is displayed and works correctly but the submit button is not displayed (meaning that f.actions has no effect).