Maybe the tank is just not a large hollow structure but contains fins/compartments/whatever to restrict the sloshing motion and it's not that big a contribution to the overall motion.
If it's no stronger than a sudden wind gust, it's just something the controller has to be able to take care of without a heads-up.
If it's no stronger than a sudden wind gust, it's just something the controller has to be able to take care of without a heads-up.