Im also in the same situation,
whats happening is it is measuring the length of the thumbstick,
so if you push it down it will measure the length as 0,
then it will multiply your angle by 0, which would equal 0.
And 0 is horizantal in radians.
I am also wondering how to control the rotation by the thumbstick to aim my characters gun,
i was about to ask a similar question with other problems.