Isn't that why they tried to get Annie to go into the underground area? If she entered she wouldn't be able to transform, or at least that's what I got out of that chapter.
Wait what? I totally missed this...
Also priest telling people to block the hole made in the wall to prevent the titan from being active by sunlight or something?
Although then I wonder how Ymir transforms at the Castle since wasn't that at night too?
Armin's suspicion is confirmed when she smiles in the same way as the female titan, and by her refusal to enter the passage, where she cannot transform.