How to reproduce:
1. Open the user's attached project "SerializedReferenceBug.zip"
2. Right-click on the Project window and click on the appeared context menu's "Create" > "MySo" item
3. Select the "New My SO" file
4. In the Inspector view, click the "Add MyDerived instance" button
5. Click the "Add MyOtherDerived instance" button
6. Observe Reference List > Element 1 > Enum Field field's and value's types

Expected result: Enum field is displayed as an integer value in the text field
Actual result: Enum field is displayed as a dropdown list with string values

Reproducible with: 2019.4.19f1, 2020.2.3f1, 2021.1.0b4, 2021.2.0a2
Not reproducible with: 2021.2.0a3
Could not test with: 2018.4.31f1 (Serializable is not introduced yet)

Notes:
- Not reproducible when Reference List is collapsed
- First used derived object's type ("MyDerived" or "MyOtherDerived") will always show an enum field as a dropdown list

Workaround:
- Click somewhere in the Editor and then select a "New My SO<...>.asset" file in the Project window. Enum fields will become dropdown lists with string values