After you die and respawn in a survival world, all entities (passive mobs, hostile mobs, dropped items, etc.) turn invisible. However, you can still pick up dropped items and hostile mobs are still able to attack you. Also, if you were to drop a new item it would be visible. Exiting the world and reopening does temporarily solve the issue, but if you die again all the entities turn invisible again. Dying more than once without reloading the world results in some entities being visible and some invisible. I tested this in a super-flat creative world as well (with slimes spawning everywhere), and if you die in creative mode using commands and switch to survival mode, you get attacked by invisible slimes, but are still able to see slimes in chunks that had to be reloaded. In both instances, all invisible entities that are in front of you will reappear as soon as the death screen appears.