It's true Lisanna getting sucked in by a microanima would have been more believable if we had earlier heard rumours of mages disappearing without an explanation. However, from the beginning this series has been incredibly episodic and the lack of foreshadowing is its trademark.

I already thought I wrote a sufficient psychological explanation for why Lisanna couldn't say anything. I guess it didn't convince DE at all. I personally think it tells a lot about her personality that after two years of fighting for her life alongside the Edolas Fairy Tail she couldn't just suddenly say: "Screw you, guys, I'm going home. You keep fighting on your own."