Rails session store stopped working correctly

I have a Rails 3.2 app that uses session store in the controllers to get the user back to the screen they were previously on.

It's been working fine for over a year. All of a sudden, the production version on Heroku, has started having issues. The user is looking at a worequest and clicks on the following in order to add a comment.

<%= link_to 'New Comment', new_comment_path(:worequest_id => @worequest.id), :class => 'btn btn-primary' %>

This is the code I use:

 def new
    @comment = Comment.new
    @comment.build_attachment
    @worequest = params[:worequest_id]
    session[:return_to] ||= request.referer

    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @comment }
    end
  end

  # POST /comments
  # POST /comments.json
  def create
    @comment = Comment.new(params[:comment])

    respond_to do |format|
      if @comment.save
        if session[:return_to] != nil
          format.html { redirect_to session.delete(:return_to), notice: 'Comment was successfully created.' }
          format.json { render json: @comment, comment: :created, location: @comment }
        else
          format.html { redirect_to :back, notice: 'Comment was successfully created.' }
          format.json { render json: @comment, comment: :created, location: @comment }
        end
      else
        format.html { render action: "new" }
        format.json { render json: @comment.errors, status: :unprocessable_entity }
      end
    end
  end

In development and staging (on Heroku), the user goes back to the worequest after entering a new comment.

Now in Production, the url looks like this:

mywebsite/comments instead of mywebsite/worequests/639

I'm not even sure where the session[:return_to] gets stored. Therefore, I'm having trouble debugging the issue.

Thanks for your help!!