Portals not linking in the y-direction correctly.
x : Overworld x, x_n: Nether x
y: Overworld y, y_n: Nether y
z: Overworld z, z_n: Nether z

I placed two over world portals in "close" proximity (approx: 100 blocks). If the nether portals are place at the extact coordinates to link (x_n = floor(x/8), y_n = floor(y/2), z_n=floor(z/8)) I come out of unexpected portals, because it appears the actual calculation is (x_n = floor(x/8), y_n = floor( y), z_n=floor(z/8)).

It appears someone forgot to divide the y direction by two when determining the distance to the closest nether portal.

I can provide a test world if you want. I work around it by making sure the portals I want are closer in the y direction then the one I don't want.