I know this sounds weird, but after about 1 a week of testing on a muliplayer SMP. When I open my Recipe Book and going through nether portal, I am falling through the void, but when I have my 'Recipe book' closed, I didn't fall / fall less to the void.

I'm putting this here, so that people with better computer specs than me, that are falling to the void when going through nether portal can test this extensively