How to reproduce:
1. Open the attached project
2. Open SampleScene
3. Select GameObject in the Scene Hierarchy
4. Switch inspector to Debug mode (click the dots on the top right corner of the Inspector > select Debug)
5. Observe MyScript has a private list My Private List of ints
6. Try clicking the arrow to expand the list
7. Select MyFile in Project assets
8. Observe MyFile has a list MyList
9. Try clicking the arrow to expand the list

Expected result: You can expand the list to monitor the list values
Actual result: You cannot expand the list

Reproducible with: 2020.2.2f1, 2020.2.5f1, 2021.1.0b4, 2021.1.0b8, 2021.2.0a6
Not reproducible with: 2018.4.30f1, 2019.4.21f1, 2020.2.1f1, 2021.1.0a1, 2021.1.0b3

Workaround: Use [SerializeField] to display values in Normal Inspector mode, but this will allow modifying the values

Notes:
1. MyFile MyList field in inspector can not be expanded to inspect content in Normal mode as well