It is understandable for peaceful unions but it just doesn't make any sense for forced unions. Junior partners shouldn't be able to break away on ruler death until the truce ends.
Yeah the truce part in particular is the really BS part of it. You're going to tell me that despite having just crushed the enemy nation with 100 warscore. Shattered their armies, and destroyed their navy, that when my king dies 2 weeks later they're just going to immediately declare independence despite not having any troops? Giving you the option to either truce break, or lose the union entirely, as more often then not the truce is longer then the CB's duration.
I mean, this is pretty much exactly what happened with Henry V of England and his victory over France just before game start. He died 2 years later before he could secure himself/his son on the throne and the French nobility rallied against his son, restarting the 100 Years War. (There were a few complications, since the French King was still alive, but Henry had beat him militarily, the French King was sickly, and Henry had been named his regent and heir, so effectively a PU-like situation).
During too early after securing a PU and especially a forced one should have a decent chance of the local nobles putting one of their own or a distant relative of the old king in charge. Time is needed to secure that kind of succession, and you do get a candid belli to re-enforce your claim. I do agree though that the truce should be cancelled in this case though.