I have a portal in the over world.
I reset the nether so there is no longer a portal in the nether. 
I built the portal in the nether y=115, and went through the portal.
That brought me to my over world portal.
Then when I went back through my over world portal, it brought me to a completely different elevation in the nether and did not create a new portal.
This is happening to everyone on my realm.