They can still slowly spin on "full" brake.
Braking torque is proportional to current, which if shorted is proportional to the motors back-EMF.
At zero RPM there is no back EMF voltage, so no braking force. Motor spins a bit, starts to generate voltage and then braking force increases.
At some low RPM braking force will be equal to the torque from the prop due to aerodynamics and it stabilizes.
(Actually approaches it from the other direction for us, but same idea applies)