Noticed it previously, had to get 2 full barrels of seeds to prove it. Started with 2 barrels containing 10000 flax seeds each, one empty trough, and one trough at about 860 units of fodder inside it.
Used one barrel on the trough containing 860 units, barrel now empty, trough now full. should have only used 4300 of the seeds.
Used second barrel on a completely empty trough containing 0 units, barrel now empty, trough now full. Worked as expected.