Its funny how stingy people are so willing to manipulate the law. It is a federal crime to disable a safety system on an automobile.
You are right about whats happening. Whenever the light is on the system has recognized a fault and disabled the system. From your symptoms you are also probably right that it is a wheel speed sensor. The car does a self check at 10kph. Above that speed faults are registered. Below the system reacts to improper sensor signals with activations.
Wheel speed sensors increase the amplitude of their signals with speed and often a poor one becomes sufficient by the test speed. In a case like yours it seems marginal and sometimes its accepted and sometimes not.
A simple test with the proper equipment identifies the culprit. Otherwise replace both front sensors. It seems to me the right one goes most often, but that could be just a Florida thingy
