diff --git a/.rubocop.yml b/.rubocop.yml
index 8d8536c8267..eec10fd71e3 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -1,10 +1,10 @@
 inherit_from: .rubocop_todo.yml
 require:
+  - ./test/action_controller_flash_before_render.rb
   - ./test/safe_navigation_cop.rb
   - rubocop-performance
   - rubocop-rails
   - rubocop-minitest
-  - ./lib/custom_cops/action_controller_flash_before_render.rb
 
 AllCops:
   Exclude:
diff --git a/app/controllers/api_keys_controller.rb b/app/controllers/api_keys_controller.rb
index 0c534bff618..f30ac8ab401 100644
--- a/app/controllers/api_keys_controller.rb
+++ b/app/controllers/api_keys_controller.rb
@@ -21,7 +21,7 @@ def create
     @api_key = ApiKey.new(build_params)
 
     if @api_key.errors.present?
-      flash[:error] = @api_key.errors.full_messages.to_sentence
+      flash.now[:error] = @api_key.errors.full_messages.to_sentence
       @api_key = current_user.api_keys.build(api_key_params.merge(rubygem_id: nil))
       return render :new
     end
@@ -32,7 +32,7 @@ def create
       session[:api_key] = key
       redirect_to profile_api_keys_path, flash: { notice: t(".success") }
     else
-      flash[:error] = @api_key.errors.full_messages.to_sentence
+      flash.now[:error] = @api_key.errors.full_messages.to_sentence
       render :new
     end
   end
@@ -50,14 +50,14 @@ def update
     @api_key.assign_attributes(api_key_params)
 
     if @api_key.errors.present?
-      flash[:error] = @api_key.errors.full_messages.to_sentence
+      flash.now[:error] = @api_key.errors.full_messages.to_sentence
       return render :edit
     end
 
     if @api_key.save
       redirect_to profile_api_keys_path, flash: { notice: t(".success") }
     else
-      flash[:error] = @api_key.errors.full_messages.to_sentence
+      flash.now[:error] = @api_key.errors.full_messages.to_sentence
       render :edit
     end
   end
diff --git a/app/controllers/multifactor_auths_controller.rb b/app/controllers/multifactor_auths_controller.rb
index 1ddfe2469b3..761ba2e3a77 100644
--- a/app/controllers/multifactor_auths_controller.rb
+++ b/app/controllers/multifactor_auths_controller.rb
@@ -19,7 +19,7 @@ def create
       flash[:error] = current_user.errors[:base].join
       redirect_to edit_settings_url
     else
-      flash[:success] = t(".success")
+      flash.now[:success] = t(".success")
       render :recovery
     end
   end
diff --git a/app/controllers/owners_controller.rb b/app/controllers/owners_controller.rb
index 560cb753fbc..f1a9d4088a3 100644
--- a/app/controllers/owners_controller.rb
+++ b/app/controllers/owners_controller.rb
@@ -77,7 +77,7 @@ def notify_owner_added(ownership)
 
   def index_with_error(msg, status)
     @ownerships = @rubygem.ownerships_including_unconfirmed.includes(:user, :authorizer)
-    flash[:alert] = msg
+    flash.now[:alert] = msg
     render :index, status: status
   end
 
diff --git a/lib/custom_cops/action_controller_flash_before_render.rb b/test/action_controller_flash_before_render.rb
similarity index 63%
rename from lib/custom_cops/action_controller_flash_before_render.rb
rename to test/action_controller_flash_before_render.rb
index 632d65c28af..045cbf0a739 100644
--- a/lib/custom_cops/action_controller_flash_before_render.rb
+++ b/test/action_controller_flash_before_render.rb
@@ -6,6 +6,10 @@ module Rails
       # Using `flash` assignment before `render` will persist the message for too long.
       # Check https://guides.rubyonrails.org/action_controller_overview.html#flash-now
       #
+      # @safety
+      #   This cop's autocorrection is unsafe because it replaces `flash` by `flash.now`.
+      #   Even though it is usually a mistake, it might be used intentionally.
+      #
       # @example
       #   # bad
       #   flash[:alert] = "Warning!"
@@ -18,9 +22,9 @@ module Rails
       class ActionControllerFlashBeforeRender < Base
         extend AutoCorrector
 
-        MSG = 'Use `flash.now` before `render`.'
+        MSG = "Use `flash.now` before `render`."
 
-        def_node_search :flash_before_render?, <<~PATTERN
+        def_node_search :flash_assignment?, <<~PATTERN
           ^(send (send nil? :flash) :[]= ...)
         PATTERN
 
@@ -31,15 +35,17 @@ class ActionControllerFlashBeforeRender < Base
         RESTRICT_ON_SEND = [:flash].freeze
 
         def on_send(node)
-          expression = flash_before_render?(node)
-
+          expression = flash_assignment?(node)
           return unless expression
 
           context = node.parent.parent
-          return unless context.descendants.any? { render?(_1) }
+          return unless context
+
+          siblings = context.descendants
+          return unless siblings.any? { |sibling| render?(sibling) }
 
           add_offense(node) do |corrector|
-            corrector.replace(node, 'flash.now')
+            corrector.replace(node, "flash.now")
           end
         end
       end