How to reproduce:
1. Download attached project file
2. Select "Test.test" in the project Assets Window
3. Check the inspector. It has "Option" property that is defined in the importer class as an enum variable (TestEnum type).

Actual result: EditorGUI.PropertyField and EditorGUILayout.PropertyField shows a popup for enum properties when used in a custom inspector (custom editor class that implements UnityEditor.Editor). However, they show an int field (number input field) when used in ScriptedImporterEditor.

Reproduced with: 2017.3.0b7, 2018.1.0a2
Not reproduced with*: 2017.1.2p2, 2017.2.0p1, 2017.3.0a7

Regression since: 2017.3.0b1