AssetDatabase.AddObjectToAsset is not persistent?

I am using OnPostprocessModel to generate some new meshes and add them to an existing asset. I use the following line to do so:

AssetDatabase.AddObjectToAsset (filter.sharedMesh, assetPath);

filter is a mesh filter generated in OnPostprocessModel and sharedMesh is the mesh to be saved.

The mesh is saved and everything works fine. However, when I close the Unity Editor and then return, the mesh is gone and all GameObjects that referenced it now have their mesh set to null. Is this a bug or am I missing something?