Virtual camera prefab continuously updated on clicking on the Solo button in prefab mode.

Steps to repro:
1. Create a new project.
2. Windows > Package Manager> Cinemachine (2.6.0) > Install.
3. Create a Virtual Camera from Cinemachine menu.
4. Create prefab of Virtual Camera(CM vcam1) > Open in prefab mode.
5. Click on "Solo" button.

Actual Result:
Virtual Camera(CM vcam1)prefab continuously updated.

Expected Result:
Virtual Camera(CM vcam1) prefab updated once.

Reproducible in:
2020.2.0a9, 2020.1.0b8, 2019.3.0f1, 2019.2.0f1, 2019.1.0f1, 2018.4.0f1 with Package version 2.6.0, 2.5.0, 2.4.0.

Working Fine in:
2018.4.0f1 with Package Version 2.3.4

Environment:
Occurring on Windows 10 & Mac 10.15.