After creating the big portal and accessing it my other Portal beneth the big portal broke and lead me to the same exit as the big one.
Any Portal created after the big one also lead to the same portal exit
All the portals in the nether lead to the same overworld portal as well (tried it with portals created far from each other)
Seams to be a coordinate or directinal problem, because other portals I lit and unlit would take the role of the exit-portal if they are closer to the worldspawn, than the previous exit-portal

(I call exit-portal, where I come out in the other dimension(seen from the dimension I was)

only one Pair of Portals (the nether-exit of the big portal; and the lit overworld-portal closest to the worldspawn) takes you to each other (until something closer to the spawn gets lit-up)

Sadly couldn't figure out why it did occure. I tried on a new-created flat world, but the portals are working fine on the new one.

The bugged portals are in a world created in 0.12.1 (the same version)