• Server OS: Mohist
• 1.12.2 Forge
  Last Version of REID
• 0 mod installed

step to reproduce:
start a server with REID and FAWE (fastAsyncWorldEdit)
do a command to edit the world, then execute //undo

• No error on the console
• No error on client
  The undo command just replace all block in area to air without
  this happen:

https://media.discordapp.net/attachments/1224013658707722330/1242785054053367849/image.png?ex=664f1946&is=664dc7c6&hm=4ed0d10443096b2856f9a48065143428300313c4a74f3cf8ee3b4a8042cecda6&=&format=webp&quality=lossless&width=720&height=289

I don't plan on supporting FAWE, as it looks like there is no easy way to get the source in a dev environment. Please use regular WorldEdit (the Forge mod version was tested, not the Bukkit plugin), as that is supported by REID. In general, non-Forge servers aren't really supported either.