furnace_smelt item modifier does not work on entities

The furnace_smelt item modifier only works on blocks, not on entities. I used the following item modifier:

{ "function": "furnace_smelt" }

to modify a clay ball on both a chest and the player. In both cases the command says that the slot was replaced with a brick, but only on the chest was this actually the case. I also tested it on armour stands and chest minecarts, both of which give successful command feedback but do not actually modify the item.