on my server, I went into the nethers and hit a zombie killed a pig that had wandered in there. I break a bunch of blocks to get around and decide to go back to the over world. I step out of the portal and turn around. Just the bottom 2 nether portal blocks were there and working, the others were not there and you could go through without teleporting. I am in creative and was the entire time. I know there was a similar bug but this is not the same.