I built a Nether portal normally in the Overworld, then entered the Nether. When I came through, a portal was not generated in the Nether. Players and items instead enter at a particular point in the air. If blocks are placed at this point, players just suffocate. Since the entrance point to the Nether is in the Crimson Forest, perhaps it's some related to the new biome generation of the Nether?

This can't be reliably reproduced, but if the restriction on upload size is changed to 50 MB, I can upload a save with the affected portal at 423/75/171 (I'll forget the co-ordinates, so I'm writing them at this point to save myself trouble later).