Yeah this is a known behavior at the edges of your sensors, even FarCom. It will not always immediately clear old signals from robots passing across the edges which can sometimes lead to momentary "ghosting" along that line. It updates normally once a turn has passed, or any other action that forces an update. (The main architectural reason being it involves too much extra checking to ensure the signals are always precise in those regions, so at present some fuzziness is allowed.)
