I wonder how they're gonna explain changing the future, after all they said "for the first time". Which means previous attempts to change the future seen in clairvoyance results in failure.
Now here's my crackpot theory, by observing the future we reinforce it. How this clairvoyance magic works is beyond me but if they observe the future state of objects then it might decide the state of the object in that instance of time they observed and alterations to the present will still inevitably reach that outcome. AKA, deciding the result before the process happens, now no matter what happens in the process the result stays the same.
A theory on why Solo can change the future is that Solo has some sort of nullification ability and that somehow the thing that "locks in" a future state is magic, perhaps the clairvoyance magic itself and that somehow even after a future state is observed, if it isn't 'magically bound' the state continues to flow freely. There's a leap of logic since we were talking about the quantum mechanic of future sight to rationalize it then wedge a macguffin magic explanation there but it's an interesting thought also, I'm theorizing in literally the first chapter so assumptions like Solo having a nullification ability could just turn out to be not true. A less interesting theory is that Solo is just 'special' in whatever convenient way that would be.