I have a fairly new world on Minecraftserver.net and after I restarted it after 1.12 launched there are some chunks that are bad.
Chunks 11 4 -11, 6 4 -12, 4 4 -12, 6 4 -6, 6 4 -5 are Extreme Hills+ biomes in a Plains biome. I have also noted that 8 4 -5 is become a swampland instead of plains. If I mine or change chunks they are reset after logging out.
My seed is -9095450570362222980