Ok, I have just tested myself: The same thing happens to me, if I only assign one axis to "yaw", but no other axis to "left brake" and "right brake".
Iirc Austin talked about this - as a mean to "help" people turn the aircraft if they can´t use brake pedals. Some smaller aircraft need to use the wheelbrakes to help turn the aircraft...
I am not sure when they implemented this, but I will file an (angry) bug report right now.
For you the only option right now is to live with it, or assign some other axis to "wheel brake left" and "wheel brake right". Ideally get some rudder pedals with brakes, they are pretty much essential for realistic flying, anyway.
I will let you know what they say,
Jan