Serialized enums, that derive from abstract classes are shown as integers in the Inspector.

Steps to reproduce:
1) Open the latest attached project.
2) Open 'TestScene'.
3) In the Hierarchy, select 'Main Camera'.

The script attached to Main Camera has two serialized enums - SingletonType and SingletonType2. SingletonType is defined in an abstract class and derives from it. SingletonType2 is a regular enum. When they are set to be shown in the Inspector, using the [SerializeField] attribute, SingletonType shows enum values as a dropdown list, but instead of SingletonType2 values, a simple integer field is shown.

Reproduced on:
2017.1.0a3, 5.5.2p2, 5.2.4f1