Use field_for_person

app/models/form/sales/pages/person_known.rb

@@ -10,7 +10,7 @@ class Form::Sales::Pages::PersonKnown < Form::Sales::Pages::Person
 
   def questions
     @questions ||= [
-      Form::Sales::Questions::PersonKnown.new("details_known_#{@person_index}", nil, self, person_index: @person_index),
+      Form::Sales::Questions::PersonKnown.new(field_for_person("details_known_"), nil, self, person_index: @person_index),
     ]
   end
 

app/models/form/sales/questions/person_known.rb

@@ -10,7 +10,7 @@ class Form::Sales::Questions::PersonKnown < Form::Sales::Questions::Person
     @hidden_in_check_answers = {
       "depends_on" => [
         {
-          "details_known_#{person_index}" => 1,
+          field_for_person("details_known_") => 1,
         },
       ],
     }