Hi !

If their is air under the portal in the Overworld (like put the portal on a platform in the air) and you return into the Overworld, you will get teleport under
If their is block above the portal in the Overworld (no matter the distance) and you return into the Overworld, you will get teleport above all those blocks

Minecraft 1.18.2
Fabric 0.14.9
Mod 6.0.0