Nether portal no longer leads to the overworld portal it was linked to, but instead creates a new portal about 100 blocks away. This happens with my first nether portal in the world and the automatically generated counterpart in the nether. There are no other nearby portals in the overworld. The overworld portal is unbroken, but sometimes the nether portal will create a new portal instead of remaining linked to the original. It has happened twice in about 10 trips to the nether.