One form element not being rendered when validation fails - ruby-on-rails

Question

I have not been able to figure out if this is a simple_fields_for problem, a cocoon issue, or something else. If I have blundered with a </div> placement, I don't see it.
When the form first displays, it renders an input field for protocol name. The user can click buttons to add form elements of interest. This works fine and looks like this:
Here's how it looks after the user has clicked on each of the "Add an element" buttons:
The user can add 0..many of each of the elements. When they click on 'Save' it all works really well.
If there is a validation error in one of the fields, the form re-renders fine with one exception. Validation errors for the "Imaging Step" elements do not re-display at all. The other elements re-render and are highlighted as expected when validation fails.
Here's a pictorial example. The user fills out part of the form, having forgotten to select a "Sequence" and having forgotten to enter text for "Tip Description":
After clicking 'save' and failing validation, the form re-renders like this:
As you can see, the Imaging Step section has not been redrawn.
If I look at params in the context of the view, everything seems to be there. #protocol.errors looks right to me as well. Models seem OK too.
Here is a pastebin of the form code.
Here is a pastebin of _step_item_fields.html.erb.
Here is a pastebin of _tip_fields.html.erb.
Here is a pastebin of my Gemfile.
UPDATE:
if I build a step_item like this:
<div id="step_items">
<%= f.simple_fields_for :step_items, #protocol.step_items.build do |si| %>
<%= render 'step_item_fields', :f => si %>
<%end%>
</div>
The Imaging Step section is always drawn, but (obviously) does not populate when validation fails. This also confuses the feature of letting the user add/remove 0..many Imaging Steps.
I also tried:
<div id="step_items">
<%= f.simple_fields_for :step_items, #protocol.step_items.build(protocol_params) do |si| %>
<%= render 'step_item_fields', :f => si %>
<%end%>
</div>
... and other variants when protocol_params is available, but ran in to forbidden params issues.
UPDATE2:
I also tried building a hash with params for a step_item. I can use this here:
<div id="step_items">
<%= f.simple_fields_for :step_items, #protocol.step_items.build(some_ok_params) do |si| %>
<%= render 'step_item_fields', :f => si %>
<%end%>
</div>
... but only for a single step_item. Am not sure how to pass a hash of hashes reflecting the 0..many functionality. Also, building a step_item this way populates the form element correctly, but does not include the error highlighting styling. This is about the time I started wondering why the simpler solution was not working.

Answer 1

Since StepItem is a sub-type of the STI class Step, I think the error detection for StepItems was getting missed. My hack fix here is not pretty and admittedly brittle. Please post a better way if you have one.
I changed this in _form.html.erb:
...
<%if params["protocol"] && params["protocol"]["step_items_attributes"].present?%>
<%params["protocol"]["step_items_attributes"].each do |sia|%>
<%v = sia[1]%>
<div id="step_items">
<%= f.simple_fields_for :step_items, #protocol.step_items.build(:orientation_id => v["orientation_id"], :sequence_id => v["sequence_id"], :note => v["note"], :id => v["id"], :protocol_id => v["protocol_id"], :institution_id => v["institution_id"] ) do |si| %>
<%= render 'step_item_fields', :f => si%>
<% end %>
</div><!-- end step_items-->
<%end%>
<%else%>
...
I changed this in _step_item_fields.html.erb:
...
<div class="nested-fields">
<div class= "form-inputs">
<div class="row">
<div class="col-sm-2 text-right">Imaging Step:</div>
<div class="col-sm-2 drop_col ">
<%= f.collection_select :orientation_id, Orientation.where("institution_id=?",current_user.current_institution_id),:id,:name, {:prompt => "Pick an Orientation", }, {class: "form-control #{"dropdown_error" if f.object.orientation_id.blank? && (f.object.sequence_id.present? || f.object.note.present?)}"}%>
</div>
<div class="col-sm-2 drop_col ">
<%= f.collection_select :sequence_id, Sequence.where("institution_id=?",current_user.current_institution_id),:id,:name, {:prompt => "Pick a Sequence"}, {class: "form-control #{ "dropdown_error" if f.object.sequence_id.blank? && (f.object.orientation_id.present? || f.object.note.present?) }"}%>
</div>
</div>
<div class="row">
<div class="col-sm-2"></div>
<%= f.input :note, :wrapper_html =>{:class => 'col-sm-8'}, label: false, placeholder: 'You can put an optional note here.' , :input_html => {:size => 50}%>
<%= f.input :institution_id,:as => :hidden, :input_html => {:value=>current_user.current_institution_id} %>
<%= f.input :id,:as => :hidden %>
<div class="col-sm-2">
<%= link_to_remove_association "Remove Imaging Step", f , :class=>"btn btn-danger btn-xs placemid",title: "Click here to delete this imaging step.", data: {toggle: "tooltip", placement: "auto", animation: true, delay: {show: 700, hide: 100}}%>
</div>
</div>
</div>
<div class="col-sm-12"><hr></div>
</div>
...
And I added this to the correct stylesheet:
.dropdown_error{
color: red;
}
The form now displays StepItem errors correctly as seen here: