The comet brightness fell suddenly just as the sky cleared off. I should say that this sort of behavior is to be expected of comets. As the approach closer to the Sun, gasses evaporate straight from solid to gas (“sublimation” it is called) and the expanding cloud reflects more light than the chunk of rock and ice alone. These gasses get blown away by the Solar wind, and the comet dims again. If this did not happen, the comet nucleus would simply reflect sunlight and its “light curve” would look like the green dotted trend I calculated. I added the observed brightness for Feb 4 – which happens to fall near that curve.
Sorry to have wasted your time. – Steve