Expected behaviour:

 https://minecraft.gamepedia.com/Nether_portal#Behavior

Actual behaviour:

When building a Nether portal in the Overworld, a Portal in the Nether gets generated, but when you create another portal in the overworld they are all linked to the same portal in the Nether. This also means that when you try traveling back trough the portal in the nether you get transported to the first build Nether portal on the overworld, no matter where you entered initially. 

Found in:

Minecraft:Java - Singleplayer
Minecraft:Java - Server

How to reproduce:

1. Build a nether portal in the overworld and travel trough it
2. Build another nether portal in lets say 1000 blocks distance to the first nether portal in the overworld
3. Travel trough the second portal and back, you will land at the same portal in the nether and when traveling back you spawn on your first portal in the overworld