How to reproduce:
1. Open the attached project
2. Open "Masked Inner Canvas" scene
3. Expand Canvas>Mask Panel and select Inner Canvas
4. Enable "Override Sorting" and notice that Inner Canvas text completely disappears
5. Enter play mode and observe how the left nested canvas changes, when it is set to override the sorting order
6. Exit play mode and observe that its visual change persists

Note: In contrast, when the masking parent uses a Rect Mask 2D, the inner canvas is always masked. It doesn't matter whether it overrides the sorting order or not. The expected behavior is for the child canvas to no longer be masked

Not reproducible: 5.3.4p3