New world generation clashing with old generation

Please provide the pre-upgrade world file.

The file is over 10mb, how would I go about sending it to you?

You can use any cloud service of your choice, e.g. Google Drive, Dropbox, OneDrive, Mediafire, whatever.

Just making sure there wasn't a preferred page
https://www.mediafire.com/file/un6kdr7hqzrpkyf/TruSiMP+(Saved+chunks).zip/file

Looks like this world was loaded in bukkit at some point, does this happen with an unmodded world?

The blending is better, kinda odd that I keep getting surrounded by glaciers, lol. Also, that's funny that it only happened to a Bukkit server when my plugins were as follows

AnvilLot (Lets you see how much XP is "Too Expensive!"

CoreProtect (Lets you see changes in a world like someone stealing from a chest)

Geyser/Floodgate (Lets Bedrock players join the Java server)

GSit (Lets players... uhh... sit)

ImageMaps (Lets you make custom images with maps)

WorldEdit (Lets you change blocks within a specified region)

I wouldn't think any of these would affect how the world generates, considering I was able to make new regions/chunks when upgrading to 1.17 with the official Caves and Cliffs preview datapack, and the new resulting generation looked just fine, only real issue being trees getting cut off and being able to walk into the void if you were in the new chunks underground

SOLUTION FOUND

First, you need to remove the Caves and Cliffs datapack. Loading the world in 1.18 in safe mode still messes up the world generation.

Next, you need to render all of the region borders that are important to you. You don't need to do every side of each region, just the borders that are next to the chunks you want regenerated. I assume this is so that 1.18 has context on where it needs to start generating new stuff without that nasty cutoff line

Blending with experimental datapack is not supported

Yup. The old caves & cliffs data pack is outdated and unsupported and will break things. (And worlds that ever had it are unsupported too)