Don't use implicitly build custom_values for change detection

app/models/custom_value.rb

@@ -61,6 +61,10 @@ class CustomValue < ApplicationRecord
     end
   end
 
+  def default?
+    value == custom_field.default_value
+  end
+
   protected
 
   def validate_presence_of_required_value

lib_static/plugins/acts_as_customizable/lib/acts_as_customizable.rb

@@ -256,6 +256,9 @@ module Redmine
             # Skip when the old value equals the new value (no change happened).
             next cfv_changes if value_was == cfv.value
 
+            # Skip when the new value is the default value
+            next cfv_changes if value_was.nil? && cfv.default?
+
             cfv_changes.merge("custom_field_#{cfv.custom_field_id}": [value_was, cfv.value])
           end
         end

spec/models/work_package/work_package_acts_as_customizable_spec.rb

@@ -135,5 +135,16 @@ describe WorkPackage, 'acts_as_customizable' do
     before do
       setup_custom_field(custom_field)
     end
+
+    context 'with a default value' do
+      before do
+        custom_field.update! default_value: 'foobar'
+        model_instance.custom_values.destroy_all
+      end
+
+      it 'returns no changes' do
+        expect(model_instance.custom_field_changes).to be_empty
+      end
+    end
   end
 end