Birches no longer generate in Dark Forest biome

I created a buffet world,and wanted to build with Dark Oak,Birch and Oak. But I didn't see any birches. I even tried to look for birches in creative mode, but I didn't find any. When I downgraded the version to 1.11, they were generating. Even in 1.12,it was alright. But in 1.13, I didn't find a single birch tree.

Thanks for reading.