All of the above contains useful advice.
In my experience there are sometimes occasions where it is not possible to accurately assess the effectiveness of the corrective action - there may be issues where you would have to wait for an event pattern to play out again to verify that your precautions put in place were effective.
And if that event pattern only recurs once in a blue moon, or never, you can make reasonable assumptions about the effectiveness of your action but without a test case to verify you are only making assumptions.
Other times it is likely that you'll have a chance to experience the event again in a year or so. How valuable is it to keep the CA open and review it then? To me, that depends on how much resource you have dedicated to monitoring events and linking to open CAs.
Sometimes I will close CAs with the effectiveness noted as apparent, but contingent upon no further recurrences of this issue. We can split hairs all we like, but the value of me (an army of one, small company) maintaining a list of CAs open for years and feverishly monitoring them on the chance that I may find incontrovertible evidence of full effectiveness does not seem like an effective use of resources.