[Experimental] Loading world (created before 1.17.20.23) creates a huge mountain at the exit site

Confirmed.
In my case I used an old world from 2017 and then upgraded it to 1.17.20.23 and then 1.17.30.23
Original bedrock level of the world has been raised to 64, 1.17.20.23's bedrock extends from 0 to 64. Void exists in previously (2017 and 1.17.20.23) generated chunks that have been modified (ex mined a block) in some way modified from -64 to 0. Un touched but previously generated chunks are over written with new (but correct) genration except when near a previously modified chunck.

However this does not affect a world from 2017 directly upgraded to caves and cliffs in 1.17.30.23.

I’ve found a cinematic example of this issue. I agree that it shouldn’t be like this, but I have to admit that it does look cool sometimes…

Experience this as well. My partner and I have a world we just traveled to find an ocean monument in. Weve had the experimental features on all year so it's gone through each update. This most recent well the outcome is extreme as the surrounding chunks along the entire journeys path have done this. The oceans are especially frustrating. Hoping this is a quick and easy fix for Mojang and we can continue on our unopened just in case copy of the world. Otherwise sadly this has killed it and we will need a new one 😨

Are anyone still having this issue in the 1.17.30.24?
I don't have this problem anymore.

in my old world, the full central chunks returned to their place, but the edges, alas, remained in place. I think the problem is solved and in version 1.17.30.24 it cannot be reproduced!