An entity which dies due to a player's actions will not drop the loot or experience you would expect from a player kill if they are in a different dimension to the player.

How to Reporduce:

• Link two nether portals between the overworld and nether.
• Spawn a creeper on a platform and punch it it using your fists such that it falls to it's death in the same dimension as the player.
•  Observe that the creeper's death counts as a player kill and drops experience orbs upon death.
• Now repeat the process however this time spawn the creeper infront of the portal such that it travels to the nether then falls to it's death as it enters the nether. Ensure the player remains in the overworld until the creeper dies.
•  Observe that the creeper's death WAS NOT counted as a player kill and does not drop experience orbs.