I go through the nether portal on a single player world, and the nether loads before I can even move, which results in mobs spawning and killing me before the "loading terrain" screen is even off my screen. This happens when going through both ways because nether mobs go through the portal and kills you on the other side. I see this has been happening since before 1.16 though so I don't have my hopes up on it getting patched.