It was totally out of Lucien and Adam's expectation that the old witch would come back so fast.
Before trying to control the crystal ball, Lucien had carefully checked around to make sure that there was no teleportation circle in the core section. However, he did not expect that the witch could directly cast Position Exchange on her mirrored reflection!
If Lucien and Adam chose not to destroy the reflection, she could come back as soon as she sensed that the core section was being invaded; If they chose to attack the reflection, she would immediately be able to get the message and use other ways to come back in time.
Despite the craziness of the witch, she was also a very smart sorcerer!