Unless she was protected from culling (by being in My Households) she could have been culled. The game does not care about your feelings when it comes to culling, and it doesn't warn you when you approach the limit. I'm almost always at the limit in my game and it culls willy nilly, though rarely does it cull any sims that I particularly care about, because I keep those in My Households and have the limit for that set to unlimited.
Thing is, when a sim dies, they change households automatically....to a hidden household of dead people. So even if they are protected, once they die, they aren't anymore. Though, honestly, I'm fairly certain ghosts do not follow the normal culling process and instead go through the "losing connection" process, where you have time to either strengthen their connection or not, as you wish.
So, I'm going to say a game bug...unless she may have died a ways back in time in terms of time-passed in game, and it just took you a while to notice it. In that case, she was culled. Because dead sims aren't in their protected household any longer if you don't do anything about it.
