When creating a second Nether portal in the Overworld the game does not create a new portal on the Nether side, in stead it sends you back to the first portal you made.
New portals created in the Nether DO create an Overworld-portal. However, any new portals created in the Overworld, no matter how far away, will allways send you to the closest Nether-side portal in existance.

Steps to reproduce:

1. Start a new world (Creative is easier, but the bug is present in Survival as well)
2. Build a nether portal (site 'A')
3. Travel to nether to create a Nether-side portal, make sure you remember the location (place some blocks), then go back to the Overworld.
4. Travel a significant distance in the Overworld, to site 'B'
5. Create a new portal at B, travel to Nether. And BWAM! You are send to the very first Nether-site portal you created (the one linked to A)

• Creating new portals in the Nether DO create a correct Overworld-portal
• Any portal created in the Overworld will link to the closest Nether-side portal, and not create a new portal in the Nether.

I regard this a near-game-breaking, It removes our ability to easily set up a fast-travel network in the Nether.