Nether portal teleport misplacement and loading error

When leaving the nether from a frequently used portal I was returned to an unexplored area ~1000 blocks away from the overworld portal I used before, with a new portal placed. Going back through the new portal placed me in an (also) unfamiliar part of the nether, stranding me between two places I'd never been with portals I didn't build.

 

After walking back to my original portal in the overworld, I exited the game, re-opened it , and went through again to test what happened. I couldn't see where I was, then dropped into lava and lost everything. Then I respawned at my bed near the portal, tried again, and found the portal worked correctly. No error, and it connected to the place I expected.