If you are the army leader, you can resolve this by travelling to settlements so that the lords in your army can purchase food.
I humbly submit that a pop-up suggestion to visit a settlement when a player-led army is running out of food is the best solution to the OP's problem. As is the case with lots of strategy layer stuff, the simulation is quite sophisticated, but that sophistication, and the life it brings to the simulation, are hidden from the player.
This particular AI behavior does not need fixing as much as plenty of others do. Ananda's point about it being good for the AI to occasionally not act perfectly rationally is a good one. It should also be difficult logistically to maintain large force concentrations, since armies could historically be more populous than villages they passed through or occupied. I also like the extant aspect of gaining influence by sharing food with undersupplied lords in the party.