How to reproduce:
1. Open attached project "toggle-root-canvas.zip" and scene "toggle-root-canvas"
2. In the Hierarchy window, select parent "Canvas" object
3. In the Inspector window, observe that "Canvas" component is disabled
4. In the Hierarchy window, select child "Canvas" object
5. In the Inspector window, observe that "Canvas" component is enabled and "Override Sorting" is enabled (the issue - child Canvas should be rendered/visible)
6. Enter Play mode (neither red or white cube is visible)
7. In Game View, press on "Toggle" button to enable the parent "Canvas" component
8. In Game View, press on "Toggle" button again to disable the parent "Canvas" component

Expected result: in the Game View, the red cube is always visible
Actual result: The red cube is only visible when enabling and disabling parent Canvas

Reproducible with: 2017.4.24f1, 2018.3.13f1, 2019.1.0f2, 2019.2.0a11