only doing one away rotation will never hurt you in the application process. i think it puts you at a distinct disadvantage because 1) it automatically puts you behind the people who have done more rotations, and 2) you have less perspective. perspective is important when generating your rank list. if you don’t know what’s out there, you will be blindly ranking programs (which i guess is what most people do anyway).
it will never be a red flag, though.