When the Default "External script Editor" in the Preferences is changed and set to Visual Studio for mac then Intellisense does not work

Steps to Reproduce:
1. Create a new project.
2. Unity > Prefrances >External Tool > External script Editor > Open by file extention
3. Unity > Prefrances >External Tool > External script Editor > Visual Studio for mac
4. Create C# scripts.
5. Open Script in Visual Studio 2019
6. In Start() start typing UnityEngine specific or type/variable/function (e.g. "Coll...", "game...", "Des...", )

Actual Result:
Intellisense(Auto-completion ) does not work

Expected Result:
Auto-completion should predict the rest of a type/variable/function

Reproducible on:
2020.2.0a12, 2020.2.0a8, 2020.1.0b10, 2020.1.0b8

Working fine on:
2020.2.0a7, 2020.1.0b7 2019.3.15f1

Environment:
Windows 10 and macOS 10.15