use submission completed_at

app/controllers/templates_controller.rb

@@ -14,7 +14,7 @@ class TemplatesController < ApplicationController
     submissions = Submissions::Filter.filter_by_status(submissions, params)
 
     submissions = if params[:completed_at_from].present? || params[:completed_at_to].present?
-                    submissions.order(Submitter.arel_table[:completed_at].maximum.desc)
+                    submissions.order(completed_at: :desc)
                   else
                     submissions.order(id: :desc)
                   end