What I expected to happen was...:
When going from the Nether back to the Overworld, I expect to spawn in the overworld at my portal that I made. I have not moved either the portal in the Nether or in the overworld.

What actually happened was...:
Instead, I spawn at a newly made portal. This has happened in two separate worlds for me. In one world, the new portal was made about 100 blocks away in a cave that I had never been to. In other world, The portal was about 50 blocks away on the surface. In the second world, I broke the newly generated portal (using tnt). I then went back to the nether, quickly returned to the overworld, and spawned in the correct portal (the one I made). However, the very next trip to the nether and back resulted in a newly generated portal right next to the old portal. After breaking the portal again and testing it again, another portal was generated, again, right next to the other broken two.

Steps to Reproduce:
1. Build nether portal
2. Go to the nether
3. Return from the nether

Again, I haven't found a rhyme or reason why or when a new portal gets generated, but it has happened often to me. As far as I know, the portal is not obstructed by anything. I usually build a portal with a wall on one side of it, but one side is opened.