Hello,
I wanted to create a custom flat world for my server, but found out I can't create decorations, it just removes the lines of code. I will add that in version 1.13 it worked normally
(for example)
I type:
minecraft:bedrock,2*minecraft:dirt,minecraft:grass_block,60*minecraft:cobblestone;minecraft:taiga;village,mineshaft,stronghold,biome_1,dungeon,decoration,lake,lava_lake

And I get:: 
minecraft:bedrock,2*minecraft:dirt,minecraft:grass_block,60*minecraft:cobblestone;minecraft:taiga

(Sorry for my language but I used a translator)