It appears as though entities do not react to the world when far enough from the player, about 216 blocks. This includes:
Falling
Taking Damage
Aging (for entities with ticking NBT tags)
To reproduce:
1. Create a new superflat world with no entities
2. Run /summon chicken ~ 300 ~ {Tags:["MC-144678"]}
3. Run /data get entity @e[type=chicken,limit=1,tag=MC-144678] Pos[1] repeatedly
In 19w07a and before, this would return a slowly decreasing y value, which reflects the chicken falling. However, from 19w08a, the y value is stuck at 300.
Running /summon chicken ~ -300 ~ demonstrates how the chicken will not take damage if it is summoned in the void.
Of course, chickens are used because of their slow falling speed, which makes them easy to test. However, this will work with any entity. In particular, using items or area effect clouds means that their Duration or Age, respectively, does not update.
Linked issues
Comments 3
I'm aware of how chunk loading works. This bug is primarily noticeable with the height, which is independent of chunk loading. Note that this bug necessarily affects loaded entities, because unloaded entities wouldn't be able to interface with commands at all.
Does this bug have the same cause as MC-145262 ?
"Chunks near the player are loaded into memory. The range depends on the Render Distance setting. These chunks may have activity (mobs spawning, trees growing, water flowing, dropped items disappearing etc.), while chunks outside of the range are inactive, and are stored on the disk." - https://minecraft.gamepedia.com/Chunk#Loaded_and_unloaded_chunks
There is also the rule, that entities only get processed, if all chunks of a 5 by 5 chunk grid centered on the chunk of the entity are loaded.