Align the Y coordinates of the Nether with the Overworld.

When linking portals to the overworld using Nether portals, the Y-coordinates must also be aligned, otherwise they will interfere with nearby portals and will not link properly.

The Nether Y=0~124
Overworld Y=-64~320

There is a significant gap in the Y-coordinates between the overworld and The Nether, and if you build structures above and below, you cannot create portals that do not interfere with each other.