I have the same issue. It happened right after the update, and it seems like all the blocks below y0 were replaced with air. It leads to some weird stuff like redstone dust placed at exactly y0 floating on nothing, and bubble elevators that extended below y0 stop in midair with no water extending past y0. Even though the soul sand has been deleted and a lot of the water is missing, what is left at and above y0 still pushes you up despite there being no soul sand below.
I have the same problem. It happened after dying in the end. I went back to the end after dying, and when I tried to return to the overworld I died. it didn't tell me I died from a high place, just that I died. Then the game freezes for me and gets stuck on a page saying "generating world" and I have to restart the game to spawn in the overworld minus my stuff and my levels. It happens exactly the same way without fail every time I try to go from the end back to the overworld. The portal Insta kills me and game freezes.
Weirdly this happens with some texture packs for me but not others. Hopefully a fix comes through eventually
It happens with naturally generated as well as placed blocks, and is purely a visual bug where the water is invisible and in the case of seagrass you see a 2D version of the seagrass texture on the top side of the block. You can swim through these blocks as normal, and once you break the seagrass the water renders normally again.