It is 1707. I'm playing as Venice. I started the game in 1453 with Corfu as my vassal, and I have yet to annex it. The core was supposed to have expired (in 1449) which is before I even started the game.
So it seems to be a bug to me. Something to note: At one point I decided to try taking it by dissolving our alliance, releasing it from vassilage, and declaring war on it. But doing so causes it to lose the core within a month after I released it from vassilage, which meant I lost the reconquest CB. I ended up reloading. I haven't tried diplo-annexing it yet, but I'm going to guess I'll be able to keep the core if I do it that way. My guess is the bug has to do with vassilage. It does something funny to cores held by the master country. It's been a minor nuisance since it's been draining my prestige for unheld cored territory for the past 350 years.