diff --git a/app/views/admin/settings/authentication_settings/_passwords.html.erb b/app/views/admin/settings/authentication_settings/_passwords.html.erb
index 7203c23b17f..3179782dfe5 100644
--- a/app/views/admin/settings/authentication_settings/_passwords.html.erb
+++ b/app/views/admin/settings/authentication_settings/_passwords.html.erb
@@ -56,9 +56,9 @@ See COPYRIGHT and LICENSE files for more details.
                       input_width: :small
 
 
-      form.hidden(name: "settings[password_active_rules][]", value: "", scope_name_to_model: false)
       form.check_box_group(name: :password_active_rules,
                            disabled:,
+                           include_hidden: true,
                            label: I18n.t(:setting_password_active_rules)) do |group|
         OpenProject::Passwords::Evaluator.known_rules.each do |value|
           group.check_box(value:,
diff --git a/lib/primer/open_project/forms/dsl/input_methods.rb b/lib/primer/open_project/forms/dsl/input_methods.rb
index 1ed63ce8168..1ecdb76798b 100644
--- a/lib/primer/open_project/forms/dsl/input_methods.rb
+++ b/lib/primer/open_project/forms/dsl/input_methods.rb
@@ -19,7 +19,8 @@ module Primer
             super(**decorate_options(**), &)
           end
 
-          def check_box_group(**, &)
+          def check_box_group(include_hidden: false, **, &)
+            add_input Primer::Forms::Dsl::HiddenInput.new(builder:, form:, multiple: true, value: "", **) if include_hidden
             super(**decorate_options(**), &)
           end
 
diff --git a/lookbook/previews/open_project/common/advanced_form_inputs_preview.rb b/lookbook/previews/open_project/common/advanced_form_inputs_preview.rb
index 1df46613f3c..5797a60f267 100644
--- a/lookbook/previews/open_project/common/advanced_form_inputs_preview.rb
+++ b/lookbook/previews/open_project/common/advanced_form_inputs_preview.rb
@@ -39,6 +39,22 @@ module OpenProject
       def advanced_check_box_group
         render_with_template
       end
+
+      # `check_box_group` with `include_hidden: true`
+      #
+      # HTML forms do not submit unchecked checkboxes, so when every option in an
+      # array-valued `check_box_group` is unchecked, the field key is absent from
+      # the request params entirely. The server then has no way to distinguish
+      # "user cleared all selections" from "this field was not part of the form",
+      # and the previous value is silently preserved.
+      #
+      # Pass `include_hidden: true` to emit a hidden sentinel field before the
+      # checkboxes. This mirrors the behaviour of Rails' own `check_box` helper
+      # and ensures the key is always present in params — as an empty array when
+      # nothing is checked — so the server can save the empty selection correctly.
+      def check_box_group_with_include_hidden
+        render_with_template
+      end
     end
   end
 end
diff --git a/lookbook/previews/open_project/common/advanced_form_inputs_preview/check_box_group_with_include_hidden.html.erb b/lookbook/previews/open_project/common/advanced_form_inputs_preview/check_box_group_with_include_hidden.html.erb
new file mode 100644
index 00000000000..8b7524c9534
--- /dev/null
+++ b/lookbook/previews/open_project/common/advanced_form_inputs_preview/check_box_group_with_include_hidden.html.erb
@@ -0,0 +1,22 @@
+<%
+  the_form = Class.new(ApplicationForm) do
+    form do |f|
+      f.check_box_group(name: :colors, include_hidden: true, label: "Favorite colors") do |group|
+        group.check_box(value: "red",   label: "Red")
+        group.check_box(value: "green", label: "Green")
+        group.check_box(value: "blue",  label: "Blue")
+      end
+
+      f.submit(name: "submit", label: "Save")
+    end
+  end
+%>
+
+<%=
+  primer_form_with(
+    url: "/abc",
+    method: :post
+  ) do |f|
+    render(the_form.new(f))
+  end
+%>