Reproduction steps:
1- Open the reproduction project (868994_reproduction.zip).
2- Build and run for standalone.
3- Observe all children of the nested canvas are flickering.

Reproducible on: 5.6.0b3, 5.5.0p3, 5.4.3f1, 5.3.3f1.

Platforms:
1- Reproduced on Windows Standalone, macOS Standalone.
2- Not reproducible in Editor.

Details:
1- Parent canvas' render mode should be either "Screen Space - Camera" or "World Space". Camera should be attached as well.
2- Off-screen image should be above the nested canvas in hierarchy.
3- Nested canvas should not have any visible sibling images.
4- Enabling "Override Sorting" in nested prefab stops flickering.
5- Deactivating or making visible the off-screen image, stops flickering.
6- Nested canvas should have at least one child game object with a Transform/RectTransform component, a script to move it and a Canvas Renderer. UI Image is used in the reproduction project but Image component's itself is not necessary at all.
7- Instead of step 6, moving script can be attached directly to the nested prefab. Child is only necessary for observing in such case.