How to reproduce:
1. Open the user's attached "Rechnik.zip"
2. Open the "testscene" Scene
3. Select the "Main Camera" GameObject in the Hierarchy window
4. Observe the "Test (Script)" Component's "Invisible Person Value" property in the Inspector

Expected result: The "Invisible Person Value" property (which is a class) has name and age fields shown in the Inspector
Actual result: The "Invisible Person Value" property has no fields, only the label is visible in the Inspector

Reproducible with: 2019.4.9f1, 2020.1.4f1, 2020.2.0b1
Could not test with: 2018.4.27f1 ("Test (Script)" Component is not visible in the Inspector)

Notes:
- This issue is also reproducible with arrays, structs, and Lists
- The issue is not reproducible with simple variables types (string, int, float, double) and enum