We can confirm this is a bug in the multiple biome blending logic in version 18.104.22.168.
It is corrected in current versions, this is how your project looks like now:
This issue is present in both Voxel Studio and the SDK. You will need updated versions for these two. If this is an urgent matter we can see how we get you a more recent version, but we may be one or two weeks away from that.
A workaround: You can still achieve the effect of having multiple biomes within a single Voxel Farm biome object. See how this project does it:
There are distinct areas of forests, icy mountains and deserts in this project, but it is all a single Voxel Farm biome definition. The variety is achieved by using different masks and layers.
This project is available for download here:
There is also a tutorial on how this was built:
The information about creating heightmaps and displacement maps may not be relevant, but it will provide the ideas you need for achieving different zones within the same biome object.