Trying to import multiple assets from single file

I’m modifying existing plugin that used to import single model from .vox file but after format was updated it can now store multiple models, like .fbx.
So far I’ve managed to create multiple static meshes, they show up in editor but when trying to save, editor crashes on:
check(StaticMeshOwner != nullptr);

What is weird that despite there are multiple assets visible in content browser, there’s only one ‘multiple assets’ item when clicking ‘save all’ and then crash.

Is there some official documentation on asset importing that I’m missing? Only useful thing I’ve found is built in fbx importer but it’s huge.

I’ve managed to get rid of crash when saving but still all assets are treated as one?
Saving one saves all the others, same with reimport.

Looks like every asset needs to have separate UPackage created and passed as Outer in constructor to get rid of this.