Terrain looks bad with two biomes

Greetings developers
I’m trying to figure out how I can use my builds into unique realism so I tried to do a western type terrain for it. The problem is that whenever I try to mix the terrain generator with another biome it would look really bad.
This is the mixed biome of Hills and plains

This one is just Hills

It looks very different as it has some kind of glitch. I want the first picture to be smooth rather than having holes

I’m still looking over reasons and tutorials on how to fix this but I cant find the solution I’m looking for. Please do help me figure this out, Thanks.

How does it look when the biome is set just to plains?

It looks smoother like the 2nd picture rather than having holes generated when two biomes are combined. It its kind of just a straight terrain rather than having any curves

Are you using a heightmap for this?

I’m not very familiar with heightmap so no I don’t think I’m using it for the terrain.