Versions
1.16.3

WorldEdit version:I have used both worldedit-fabric-mc1.16.3-7.2.0-beta-05 and worldedit-fabric-mc1.16.3-7.2.0-SNAPSHOT-dist independently to set the biome
Platform version: Fabric Loader 0.10.0+build.208-1.16.3

Describe the bug
Using the following command:
//chunk
//setbiome soul_sand_valley
results in a chat output stating "Biomes were changed for approximately 65536 blocks.
You may have to re-join your game (or close and re-open your world) to see the changes."
Either unloading the chunks and revisiting them or quiting the game and restarting has had no effect on the selected chunk.

Expected behavior
Expected the biome to change to the selected biome in the command.

can't repro. to clarify, it changes this thing, not terrain

That is just it. It does NOT change that "thing" that you just showed in your screenshot. I am not looking to change the terrain, only the biome itself. See for yourself https://imgur.com/a/1sw5wFPas that was already performed on the selected chunk to change it to soul_sand_valley and it still reports it as nether_wastes

well, as you can see it changed it for me, so "can't repro" holds.

Just because it works for you does not mean that it works for all. I have tried it many times and continue to have the same results time and time again saying that it was changed (via chat only) only to have MC F3 state that that was not the case at all.

sure, I'm not trying to say it works for you, I'm simply noting that the bug report isn't valid given the provided information. you can join discord if you'd like to chat or troubleshoot your problem, and we can reopen the report if you provide some information that allows us to reproduce.

As well as getting these errors in the log:
[12:04:28] [Server thread/WARN]: Tried to register asset loader 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader' with extension 'jpg' and asset class 'com.sk89q.worldedit.util.asset.holder.ImageHeightmap', but it is already registered to 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader'
[12:04:28] [Server thread/WARN]: Tried to register asset loader 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader' with extension 'bmp' and asset class 'com.sk89q.worldedit.util.asset.holder.ImageHeightmap', but it is already registered to 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader'
[12:04:28] [Server thread/WARN]: Tried to register asset loader 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader' with extension 'gif' and asset class 'com.sk89q.worldedit.util.asset.holder.ImageHeightmap', but it is already registered to 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader'
[12:04:28] [Server thread/WARN]: Tried to register asset loader 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader' with extension 'png' and asset class 'com.sk89q.worldedit.util.asset.holder.ImageHeightmap', but it is already registered to 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader'
[12:04:28] [Server thread/WARN]: Tried to register asset loader 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader' with extension 'wbmp' and asset class 'com.sk89q.worldedit.util.asset.holder.ImageHeightmap', but it is already registered to 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader'
[12:04:28] [Server thread/WARN]: Tried to register asset loader 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader' with extension 'jpeg' and asset class 'com.sk89q.worldedit.util.asset.holder.ImageHeightmap', but it is already registered to 'com.sk89q.worldedit.util.asset.ImageHeightmapLoader'

well, look like asset loaders need to be cleared when switching between SP worlds, but that's unrelated to your issue still.

On discord now. More than happy to discuss it there