None of the Rubies so much as second-glanced Ruby’s headband/bandanna thing. Which makes sense; they aren’t so bright, and each of them have their own personal uniforms. They probably just assumed it was always there, and they don’t wanna be “that Ruby” and bring it up.
Which means Ruby literally had no practical reason to wear her snapback. She’s not pretending to be human, and nobody complained about her earlier.