To be strictly honest, my only real experience with the Sims comes from the PC version. But I can't imagine that base game mechanics like this would be handled any differently.
With that said, the only explanation that I can really come up that would make any sense to me is that perhaps their hunger bar was slightly lower when it wasn't happening. Sims also have a good deal of autonomy (unless you turn it off), and simply decide things on their own sometimes, based on their needs. Thus, if they don't have one of the "Hungry" moodlets, it's possible that they simply decide that eating it themselves wasn't the intent behind cooking it. And so, they put it down until you tell them that they should eat it.
I wouldn't honestly think that this is a bug, unless they keep setting it down while they do have the "Hungry" moodlet.