My actual theory is that Subaru’s feelings pierced her immediately. She has been continually moved by Subaru’s spirit and actions thus far, and she’s already fallen for him, but she doesn’t know how to handle it. Now, Puck’s absence obviously had an effect on it, but she’s been going the gradual change of relying on someone other than herself, or Puck; hence Roswaal’s “Emilia’s extremely weak” comments.