The bug

Invulnerable item entities and XP orbs disappear / are removed from the world client-side when they take damage. However, they are still in the world server-side and once the player gets close to them he collects them.

How to reproduce (item)

1. Summon an invulnerable item entity

   /summon item ~2 ~ ~ {Invulnerable:1b,Item:{id:"stone",Count:1b}}
   

2. Ignite the block below it with a Flint and Steel and put out the fire afterwards
   → The item is not visible anymore
3. Move towards the position the item was
   → It is still there and you can collect it

How to reproduce (XP orb)

1. Place a command block with the following command

   /summon armor_stand ~ ~ ~ {Invulnerable:1b,Passengers:[{id:"experience_orb",Value:10s,Invulnerable:1b}]}
   

2. Set the top of the command block on fire and put out the fire afterwards
   → The XP orb is not visible anymore
3. Move towards the position the XP orb was
   → It is still there and you can collect it

Types of damage

• Fire
• Lava
• Cactus

Code analysis

Code analysis by marcono1234 can be found in this comment.