Two different times now when I have travelled to the nether I can see the bedrock above me as I am falling out of the world while the world is still loading before I get teleported to the correct location. It is random and not easy to reproduce, and I have never seen it before 23w18a. It is very jarring and kind of scary because I thought I was actually falling out of the world but it is really just a visual glitch. I tried to screenshot it but it fixed itself right as I tried to.

Basically what it looks like is that you are falling out of the world even though you really aren't.

Sometimes all the bedrock is visible but sometimes it's only a small bit like in the screenshot

This is by no means affected by this but it is much easier to observe if you have the Vanilla Tweaks "No Fog" pack turned on.