How to reproduce:
1. Create a new project
2. Create a new C# script in the Project window
3. Open the script with Visual Studio
4. Observe the script

Expected result: Solution Explorer shows up, script referenced to the project
Actual result: Solution Explorer doesn't show up, script not referenced to the project

Reproducible with: 2.0.1 (2020.1.0b7, 2020.1.0b10, 2020.2.0a12)
Not reproducible with: 2.0.0 (2020.1.0b6)
Could not test with: 2017.4.40f1, 2018.4.23f1, 2019.3.15f1 (Visual Studio Editor package not available)

Note: After making some changes in the script and then re-opening it, the script gets referenced to the project and Solution Explorer shows up. In 2020.1.0b7 the very first created script is losing reference to the project and the second created script will be referenced correctly. In 2020.1.0b6 updating the Visual Studio Editor and then creating new scripts will result in all of the scripts losing reference to the project.