Maybe I'm missing something here, but I have invested heavily in supply depots, farms, cattle, fish ports, etc. across my empire; however in two playthroughs the result is the same. Late game I go from around +70 food to -150 and despite disbanding armies, using import food, and building new food buildings I cannot return to a positive food supply. Also for some reason about 30% of my food buildings destroy themselves every turn late game.