Nether Portals being weird

You probably have multiple portals in the Nether that are less than ~50 blocks away from each other. When you enter a portal, Minecraft searches for portals that are in the Overworld that are near the exact coordinates of your portal. Example:

Original Overworld portal: (0,0) Original Nether portal: (0,0) New Nether portal: (50,10)

The "New Nether Portal" searches for portals in the Overworld that are near it. It finds a portal 400 blocks away at (0,0) (the Original Overworld Portal). It links up to that portal, and you exit there.

To fix the issue, just go to your target coordinates in the Overworld (in this case, (400, 80)) and set up a nether portal there, then enter it. It should link up to the New Nether Portal. I hope this helps.