Good question and one that often comes up.
All contracts or legal compacts are deemed to be perpetual unless they have a specific end-date. For example, a marriage contract is deemed to be perpetual, as are many business arrangements unless the two parties state such contract or compact will end on xx/xx/xxxx. Even diplomatic agreements are made with this same language. Yet, any party in the contract can legally and often unilaterally leave the agreement should they deem it fit to end it. They can buy out their part of the agreement, split assets, or settle their differences in other ways. The AOC and the Constitution are no different. Representatives from SC tried to purchase federal property and settle their end of the federal debt but were rebuffed by the Lincoln administration. That did not mean they were not both de facto and de jure out of the Union of States under the Constitution. The people had, through convention, determined to rescind their ratification of the Constitution. They had legally broken the compact and were now divorced (seceded) from the Union.
One point that needs a bit of fine tuning. Please be sure to reference the act of withdraw as “secession,” not “succession.” They are two entirely different terms, but often confused.
Hope that helps.