Is This Hypothesis Correct?

I have constructed a thought experiment regarding whether the future is deterministic or not.

It goes like this:

Let’s take an Observer 100 light years away from earth. Now presently, if he looks at earth then he sees the earth as it was 100 years ago. Now I, a scientist, flashes a beam of radiation from earth towards the Observer’s direction. Now, I know that the Observer, after 100 years, will see the radiation beam if he was searching beams in our direction (which I presume he was because he has been an active communicator).

Now, according to this situation, I know that after 100 years what the future of that Observer is.

Therefore, the future is deterministic
But this violates the fact that future is undeterministic, according to Quantum Mechanics.

So, then what is the possible way to resolve this paradox ?