Summoning Living entities with 0 Health and a Death time of 100 creates ghost entities that die but stay in the world and replay the death animation on world reload.

Steps to reproduce:

1. Create a new world in 1.13 prerelease 1
2. Summon a chicken or other animal with the following command

/summon chicken ~ ~ ~ {Health:0,DeathTime:100s}

1. Wait for death animation to play out. (Side note: not items are dropped either)
2. Position the character so they are looking at where the chicken was.
3. Reload the world.
4. Death animation plays again.
5. 5-10 mins later chickens will lay eggs as well

Commands are unable to target the entity, directly or  otherwise.

/say @e[type=chicken]
//returns nothing

/say @e
//Does not locate the entities affected

TNT is able to damage the entity and blocks are not able to be placed within the entity's position.

Assuming this is something to do with the DeathTime, however, it will only work with both tags specified.