Mobs transported from Nether to Overworld cannot be identified through commands

Steps to reproduce:

• Create a portal in the Nether
• Spawn a mob (Creeper, Pigman, Cow, etc)
• Push mob through portal
• Enter portal
• See mob
• type /testfor @e

Result
The outcome is "That entity cannot be found" even though the mob is clearly there. It also will present this statement for the amount of traveled mobs there are. /kill does not work either. Exiting world, and reloading seems to fix this.