Really? The Animate Undead spell says nothing about healing or heads and body parts stitching themselves back together and while I’d be ok with a weirdly artuculated pile of animated bone, actually allowing corpses to repair themselves I think is beyond the spell RAW.
And if any pile of body parts can make a zombie, whats the point of Flesh golems etc?
I think the intent is that it be a basically intact corpse, but the wording is absent of that criteria, so I don't know if anyone can claim RAW either way.
RAW is simply "humanoid corpse" becomes "Zombie" (the stat-block) and "pile of bones" becomes "Skeleton" (also the monster stat-block).
Technically, by RAW, any humanoid corpse will do, no matter how mangled. Obviously it's up to the DM to decide if that means it repairs the corpse (or if they like, to make a modified stat-block to reflect the corpses injuries) OR to decide that it doesn't work on an "incomplete" corpse. But all of that is not really RAW, it's just standard DM fiat.
RAW is just - is it a humanoid corpse? If yes, then it's a Zombie.
If we're gonna get all rules-lawyer-y, then the OP's question becomes: 1) Does a Zombie go back to being a "humanoid" corpse after it dies, like it was before it became a zombie, or does it remain an "undead" corpse? OR 2) Either way, when you "kill" a zombie, do you effectively 'destroy' its corpse?
There is no right answer to either of those. It's up to the DM again.