Agriculture has been aware of this for a while. It is more complex than that. Loaded containers need to consider balance of this ship, and grain is typically denser/heavier than what came over (most goods come over with a lot of air), so ships need to be carefully loaded, while only empty containers are easier to load because it is easier to balance the ship.

Making the above worse, even though containers are worth more in Asia, and containers of grain are worth a lot: to ship owners they get paid more for the Asia to US trip than the return trip, a ship that leaves the US unloaded (taking on ballast while unloading) and rushes back to Asia makes more money at the end of the year than a ship that waits around in LA to be reloaded with containers.