Mob corpses (with/without loot) are disappearing in relatively short time; around 30 seconds. I've lost quite a few loot because of this. I haven't found anything related to this in bugtracker. I am not sure though if I would call this a bug and that this topic belongs in a bugtracker secton, but it certainly is an issue. So, I've decided to post it here.
This has been an issue for a long time, even raid bosses despawning with loot on them, thats why a lot of raid leads will loot it all to themselves then distribute. Its annoying, and needs to be fixed but, while leveling, not that big of an issue