Rex has been keeping tabs on the important figures on the battlefield.
He was attuned to his surroundings thanks to his keen senses.
Despite battling the soldiers at the same time earlier, he could keep track of the others without a problem.
Selvaris was the first one who pulled away from the battle, followed by Eiran who was slowly pulling back while trying to not be noticed, and then lastly, Franklin. Both Selvaris and Eiran were able to get inside the Dragna Sea's bubble in time.
However, Franklin wasn't.
Not that he was too late to move but because Rex made sure that he was trapped outside.
His Mask of the Banished Eclipse has another special effect other than the Law of Foresight, and that is the disturbance of ether flow. Once Franklin was channeling his ether to dash away, Rex used that to disturb his channeling and caused a blowback.
Franklin has no idea.
It was the power of an SSS-grade Soul Artifact.