To reproduce:

1. Open the project, attached by tester (multipleObjectsEdit.zip)
2. Open "test" scene
3. Select a, b and c game objects in the Hierarchy
4. In the Inspector, change 'Test field' value
5. Select each of those game objects individually
6. Observe the 'Test field' value

Expected: multiediting a value changes it for all components on selected game objects
Actual: value is changed in only one game object's component instead all selected when using custom inspector with CanEditMultipleObjects attribute

Reproduced in 5.5.3p2, 5.6.1p2, 2017.1.0b7, 2017.1.0b8, 2017.2.0a1

By design: In order to write inspectors supporting multi selection, one needs to go through SerializedObject/SerializedProperty.