Got into the nether portal at Y=2695 with no problem. Spawned inside netherrack at Y=2695 which had to be manually broken, and no return portal in sight. And it didn't generate at 336 (~Y÷8) but rather at 120.

I couldn't find anything in fml-client-latest.log that referenced portal generation.

Edit: After I came back from the nether, I went through the portal again, and this time it spawned the portal at 2695 where I had originally been sent. This seems to be because I had dug out a space the last time. So the code appears to also require air blocks.

Could you upload a log anyway? It may be that some mixin failed to apply.

You got it.
https://pastebin.com/DQZ7fYWf
Much of it was testing the UtilityWorlds portals (those work perfectly fine now, btw).