How to reproduce:
1. Create a new project
2. Set Visual Studio Code as the Unity Script Editor (Edit -> Preferences -> External tools -> External Script Editor)
3. In the Project window create a new C# Script
4. Open the newly created Script
5. Above the Start method start writing (do not copy-paste) this line of code: 'public GameObject '

Expected result: Visual Studio Code suggests a code completion for the GameObject class name
Actual result: no code completion is suggested

Reproducible with: 1.1.4 (2019.4.0f1), 1.2.1 (2019.4.0f1, 2020.1.0b11, 2020.2.0a14)
Not reproducible with: 1.1.3 (2019.4.0f1)

Workarounds:
1. Regenerating the project files (Edit -> Preferences -> External tools -> Regenerate Project Files)
2. Creating a project with Rider or Visual Studio as the Unity Script Editor and then switching to Visual Studio Code

Notes:
The issue doesn't occur with Rider or Visual Studio