Serializer Problem with unordered groups

Searializer behaves wrong for unordered groups

serializing existing models with unordered groups and adding a new element to an existing * part leads to an invalid model

Model:
	'model' site=ID '{'
        (
            xxxxValue=XXXXValue? &
            properties+=PropertyAssignment*
        )
    '}'
;	
XXXXValue:
    "xxxx" "=" value=INT
;

PropertyAssignment:
    name=ID "=" value=INT
;

import com.google.inject.Inject
import org.eclipse.xtext.serializer.ISerializer
import org.eclipse.xtext.testing.InjectWith
import org.eclipse.xtext.testing.XtextRunner
import org.eclipse.xtext.testing.util.ParseHelper
import org.junit.Assert
import org.junit.Test
import org.junit.runner.RunWith
import org.xtext.example.mydsl3.myDsl.Model
import org.xtext.example.mydsl3.myDsl.MyDslFactory

import static org.junit.Assert.assertEquals

@RunWith(XtextRunner)
@InjectWith(MyDslInjectorProvider)
class MyDslParsingTest{

	@Inject
	ParseHelper<Model> parseHelper
	
	@Inject extension ISerializer serializer

	@Test 
	def void loadModel() {
		val result = parseHelper.parse('''
			model mymodel {
			    yyyy = 1
			    xxxx = 2
			}
		''')
		Assert.assertNotNull(result)
		println(result.eResource.errors)
		result.properties += MyDslFactory.eINSTANCE.createPropertyAssignment => [
		    name = "xxxxx"
		    value = 1
		]
		
		val ser = result.serialize
		println(ser)
		println(parseHelper.parse(ser).eResource.errors)
		assertEquals(0, parseHelper.parse(ser).eResource.errors.size)
	}

}

Start with the MyDslParsingTest reproducer and the injected ISerializer used after adding a PropertyAssignment to result.properties. Run the test with the unordered-group grammar shown in the issue, then trace why serialization produces text that parseHelper.parse reports as invalid. Done means the modified model serializes to text that reparses with zero resource errors.