When I create a nether portal and go to nether, then upon my return to overworld, another nether portal was created (as if the one made by me was not there or turned off).

To reproduce:

Create new world in superflat. Use obsidian to make nether portal, light and enter portal. Return through same portal.

Expected: Player appears in overworld at the same portal originally created by player.

Observed: Player appears in overworld at another place, at a newly created (by the game) portal.