How to reproduce:
1. Create a new Unity project
2. Go to Edit > Project Settings > Graphics and scroll down to Shader Loading section
3. Click on Save to asset
4. Reference the newly created shader variants asset in the Preloaded Shaders
5. Click on Save to asset

Expected result: Shader variants asset is still referenced in Preloaded Shaders
Actual result: Reference to Shader variants is missing in the Preloaded Shaders

Not reproducible with: 2019.3.0a1, 2019.4.8f1, 2020.1.3f1, 2020.2.0a21
Reproducible with: 2018.4.26f1, 2019.2.21f1