How to reproduce:
1. Open the attached "CustomSceneImporter.zip" Project
2. Create the material by Right-clicking Project View and selecting Create > Material
3. Create a Scene by Right-clicking Project View and selecting Create > Scene
4. Observe the difference in the Console

Expected Behaviour: All Asset Files should consistently invoke Callbacks (either both or just meta files)
Actual Behaviour: On some Asset Files Callbacks are only invoked on .Meta Files and on some on both. (.meta and the asset itself)

Reproducible with: 2017.4.29f1, 2018.4.3f1, 2019.1.7f1, 2019.2.0b6, 2019.3.0a6

Note: The difference is not only with Scene/Material, other assets may also behave differently.