How to reproduce:
1. Open attached project "Case_1211792"
2. Select AddressableTest ScriptableObject in the Assets folder in Project window
3. change the Reference variable in the inspector to "None"
4. Exit Unity and open the project again
5. Check AddressableTest asset again

Expected result: "None" value is saved to the Reference variable
Actual result: Reference value changes back to the one before setting it to "None"

Reproducible in: 1.2.2 (2018.4.15f1, 2019.2.19f1, 2019.3.0f5, 2020.1.0a19), 1.5.1 (2018.4.15f1, 2019.2.19f1, 2019.3.0f5, 2020.1.0a19)
Not reproducible in: 1.1.10 (2018.4.15f1, 2019.2.19f1, 2019.3.0f5, 2020.1.0a19)
Could not test with: 2017.4.36f1 (no Package Manager)

Notes: Changing to other values other than "None" saves them properly. The issue is not just visual, the value is actually provided to the script, which can be checked with the "test.cs" script (attached to Main Camera) which outputs the Reference's value to the Console. Does not reproduce with AssetReference. MonoBehavior scripts save the value correctly.