Sounds like it might be an issue with FBX import, have you tried importing the models into a program such as Blender and Re-exporting the models using a guide on Unity 3D model import? Unfortunately I do not use Unity so cannot offer any specific support for it.
Hello, after checking the FBX models the issue could be the included collision meshes are being mistaken for part of the model by Unity, if the assets are all weird blocky shapes this would be the issue. You should be able to work around this by importing the models into Blender as it will automatically separate the collision from the model, you can then export the models at which point they should work in Unity.
I'll have an update out within a few days to fix this issue as I was unaware of it.