This is not a bug, it's a mistake from me.
Sorry for it.

See the first comment for explanation...
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All nether portal in the overworld link to the same portal in the nether

Discovered because I wanted to travel faster with the fact that «one block in the nether = 8 block in the overworld» but it was broken

What I expected to happen when you create differents nether-portals in the Overworld (when tested, portal were at least at a thousand block from each other) is that it would create different portals in the Nether.

What actually happened is that each portal are linked to the same portal in the Nether

Steps to Reproduce:
1. Create a first portal in the Overworld
2. Go in the nether to see were you appear
3. Get back to the overworld
4. Create a second portal far away (tested at 1000 block away)
5. Enter in it and you should appear at the same place then the other portal in the nether...