After a restart, the biome is reset (MineCraft WorldEdit)?

I changed the biome of my 1.8.8 server to the Jungle biome with the command "// setbiome Jungle". However, the biome is reset after 2-3 restarts!

Why is this reset and how can I permanently change the biome?

I had the same problem in the 1.12.2.

does it have to be this area? I simply marked a slightly larger area and then it worked!