The whole concept of a reaction channel is such pretentious, unskilled, creators of nothing garbage. There is next to no chance that a reaction channel has any legal standing to show the videos they "react" to unless that permission has been explicitly granted. A fair use defense would be laughed out of court.
It is cute how they think its' ok to steal someone elses work and make a non-content video without any real effort or value added. It also means my sympathy when things go wrong is a bit on the thin side.