Don't modify params object, but use an intermediate variable

`@params_fields_to_show`

app/controllers/admin/reports_controller.rb

@@ -47,8 +47,12 @@ module Admin
       # Initialize data
       params[:display_summary_row] = true if request.get?
       if OpenFoodNetwork::FeatureToggle.enabled?(:report_inverse_columns_logic,
-                                                 spree_current_user) && request.get?
-        params[:fields_to_show] = @report.columns.keys
+                                                 spree_current_user)
+        @params_fields_to_show = if request.get?
+                                   @report.columns.keys
+                                 else
+                                   params[:fields_to_show]
+                                 end
       end
 
       @data = Reporting::FrontendData.new(spree_current_user)

app/views/admin/reports/_rendering_options.html.haml

@@ -26,7 +26,7 @@
     - if feature? :report_inverse_columns_logic, spree_current_user
       .alpha.two.columns= label_tag nil, t(:report_columns)
       .omega.fourteen.columns
-        = select_tag(:fields_to_show, options_for_select(@report.available_headers, params[:fields_to_show]),
+        = select_tag(:fields_to_show, options_for_select(@report.available_headers, @params_fields_to_show),
                     class: "select2 fullwidth", multiple: true)
     - else
       .alpha.two.columns= label_tag nil, t(:report_hide_columns)