isnt this the training for simulator?
Anyways, if it is that one:
It is because planes very far away will be seen as a obvious black dot. But in the 0-3km range, the black dot is replaced by the real plane model... making all the planes in the 1.5-3 km range very hard to see. It is no bug, its just the low detail dot, changing to high detail object.
Try to use you map to locate the plane, and fly to the place where you saw the dot. In the beginning it is very hard to spot planes, but you will get used to it.
Oh, btw, after the patch this problem should be solved
and the should no longer be a 'plane invisible range'