Dirt not spawning in custom dimensions?

So I'm trying to make a mod that adds portals to new dimensions that are only one biome, for example my first dimensions goes to the cherry grove dimension.

however everytime i go to this new dimension there is a gap between the top grass block and the stone where regular dirt would normally appear 

https://imgur.com/3R6b8Qc

Am i doing something wrong? It works fine with custom biomes, but everytime i add one from vanilla, it does this...