This was after the plague was cured: Elpis had never been told about it by the player (or even unlocked as a discussion option via Thais) before the plague was cured. You can kinda speedrun the plague cure and end up getting it well before you've told most people about the plague's existence if you're willing to throw a lot of blood at it early in a run, which is what V did! It's less jarring for most of the other people because you can imagine them having not gotten the news, but with Elpis you do actively have to go confront Howler's on the way to deliver the cure so it feels more weird that she doesn't know.
