
I think its because the way Athy said it. Cuz she was saying how she realises how important Claude is to her now and that "goodbye" was referring to the only time she could express her love honestly. Because in the orginal novel, this was where Claude left Athy for Jeanette so she thought that this could potentially be good bye forever. Plus, before she had been doing her love act solely for survival but obviously, she now geniunly loves Claude, but she believes she has to continue tricking people in order to survive. So Claude reacted that way because of he sensed that Athy was saying this so seriously and wondering why she made it sound like it was goodbye forever.
when it was athy's debut, and she was about to go dance. Why did Claude make that expression as she was disappearing into the crowd. Did he feel like a premonition that he might lose her (aka. his memories) or what?? ╮( ̄▽ ̄)╭