Caleb Truax and James DeGale are both promotional and TV network free agents that are advised/managed by Al Haymon.
The PBC head honcho will essentially make the decision for both fighters. So he'll inevitably pay the new champion a decent sum to reward him to agreeing to take the DeGale rematch (if that is something that Haymon and Chunky wants).
In this situation, I'm not sure if immediate rematch clauses are that essential.
I'm a bit of a nerd when it comes to these sort of things and I honestly cannot recall a rematch clause ever being stipulated for a bout between two fighters that were both with Al Haymon.