Since my polymorph meme has only garnered three downvotes so far I thought I'd offer a bit more controversial take, and see if I can manage to stir the pot a bit with this one.
As I've pointed out in other comments there's a specific rule for damage past 0 HP. It's called “Damage at 0 Hit Points.” under the Death Saving Throws section, and it doesn't say anything about it getting cancelled out.