Welcome back, Lisa!
They fixed Lisa disappeared a couple updates into Halloween but.... They haven't fixed it so she reappears for people yet that I know of.
Forum Discussion
They should have just built the ability to store the Simpson house into the last update. Then everyone could get her back, guaranteed, at any time. (Every character should have some sort of associated structure to store, in my opinion. Even if it's something stilly, like a bee topiary for Bumblebee Man, etc. Lisa has been the biggest issue, since she's needed for Halloween quests, but I've seen a few reports of premium or other unassociated characters going missing as well.)
