There are 2 test cases in V2AddlResponseTests fails because of this.
For example: TestCase: V2AddlResponseTests_SPEC2_12_Event_cookie5: Details: Cookies set during the Event phase should be available to the portlet during the Render phase.
According to portlet Spec 3.0 Chapter 15.1.2:
Cookies set in the response of one lifecycle call should be available to the portlet in the subsequent lifecycle calls.
In Liferay, When we want to add cookies during the event phase, we should call the EventResponseImpl.transferHeaders method at the appropriate time.