Reproduction steps:

1. Open attached project "PropertyDrawerType"
2. Open scene "Test"
3. Select GameObject "Demo"
4. In the Inspector unfold "Test Array"
5. In PropertyDrawer.OnGUI SerializedProperty.type is logged for each SerializableTest object (logged to the console)
6. Notice that if SerializableTest is in an array its type will be reported as Generic Mono instead of SerializableTest