Same bug as in Arma 2. If looking straight forward, or within a small range of degrees from 0 (looking straight forward) TrackIR 5 will zoom in.
However, If you make a head turn over a certain amount of degrees, You can no longer zoom in. In Arma2 when you try the same movement you get negative zoom, meaning, if already in a head turn, to zoom in you actually have to pull AWAY from the screen, and to zoom out you have to move your head toward the screen.
Bug report Video demonstrating the bug here - http://youtu.be/226fvWvKgV0