How to reproduce:
1. Open the attached "1291872_ReproProject" Project
2. Open the Scene "SampleScene"
3. Open Game view (Window->General->Game)
4. Open second Game view (right-click on the Game view window name->Add tab->Game)
5. Set one Game view to render Display 1 and the second Game view to render Display 2

Expected results: There's a square in the middle of both Game views
Actual results: There's a square only in the Game view which is rendering Display 1

Reproducible with: 7.2.0 (2020.1.0a11), 7.5.1 (2019.4.14f1), 8.2.0 (2020.1.13f1), 10.1.0 (2020.2.0b11, 2021.1.0a5)
Could not test with: 2018.4.29f1 because Camera Stacking was introduced in Universal RP 7.2.0 which is available in Unity 2019.3.0f6 and above

Notes:
- The Canvas is rendered on the second and other displays only when the Canvas is using the Screen Space - Overlay Render Mode