I have used multiple procedural foliage spanwers on a level. They all work well on a landscape formed of an imported heightmap. I spawn foliage by biome.
When I import another heightmap, or even copy and paste the existing one, and move it to where I want the foliage tool will not spawn on it. It remains only on the original. The landscape material is the same as is on the original landscape. The spawn area of each foliage spawner includes the new landscape.