Sending mobs through portals makes them despawn. (Nether portals from what I have tested) But just in later snapshots of 1.8. The first ones still work and 1.7 works, but not later ones. Also not even sure if this is even a bug or a really stupid feature. 😃
MC-55194
Mobs Despawn When Going Through Portals (Later Snapshots)
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Please don't put down the affected version as a future version! You DON'T have access to it yet!