Editing UnityEngine.Object fields (like Material, GameObject, etc) of a custom ScriptedImporter do not work in a default Inspector. This might be tied to the fact that setting SerializedProperty.objectReferenceValue property for these fields doesn't work. However, as a workaround, it is possible to set those fields directly through custom inspector editor, and they are correctly serialized to a meta file.

To reproduce:

1. Download attached project "BugScriptedImporterSerializedProperty.zip" and open in Unity
2. Open "BugScriptedImporterSerializedProperty" scene
3. Open "MyImporter.cs script" and observe that it has "Int", "Material" and "Prefab" fields
4. Select "asset.myasset" asset in the Project window
5. Try to edit "Int" field and observe that it can be edited
6. Try to edit "Material" and "Prefab" fields and observe that they can't be edited, nor they accept Dragged & Dropped assets

The likely reason why this doesn't work and Workaround:

1. Open "MyImporterEditor.cs" script and comment out the "#define DISABLE" line
Note: This will enable a custom editor for the asset
2. Inspect "asset.myasset.meta" file, and observe that serialized "Material" and "Prefab" fields do not reference any assets
3. Select "asset.myasset" asset in the Project window
4. Observe that first two fields showed through default EditorGUILayout.PropertyField are not editable and they don't accept Dragged & Dropped assets - just like in default Inspector
5. Observe that next two fields (custom field), showed through custom EditorGUILayout.ObjectField can accept Dragged & Dropped assets, but "materialProperty.objectReferenceValue = newMaterial;" line doesn't work, so fields remain unset
Note: Through above steps, it's impossible to modify those fields and get them to serialize to "asset.myasset.meta" file
6. Observe that next two fields (direct field), that set fields directly work correctly.
7. Set those fields, hit "Apply", inspect "asset.myasset.meta" file, and see that those fields were serialized correctly

Notes:
- This issue appears on Windows and OSX
- In Unity 2018.1 and 2018.2 this issue is fixed

Reproduced on Unity 2017.3.0a7, 2017.3.0f3, 2017.3.2f1 and 2017.4.0f1
Could not reproduce on Unity 2018.1.0b12 and 2018.2.0a5
Could not check for regression on Unity 2017.1, 2017.2 and 2017.3.0a6 because project becomes corrupted