How to reproduce:
1. Open the attached 'UIPrefabFlicker.zip' project
2. Open 'SampleScene' Scene
3. Toggle off lighting button in the Scene View
4. Zoom in and zoom out the viewport using the mouse scroll wheel

Expected result: Canvas UI GameObjects Z-Fighting when lighting is toggled off
Actual result: Canvas UI GameObjects do not Z-Fighting when lighting is toggled off

Reproducible with: 7.1.5(2019.3.0b4, 2020.1.0a17) 7.1.6(2019.3.0f4, 2020.1.0a17)
Not reproducible with: 4.1.0(2018.4.15f1), 6.9.2 (2019.2.17f1), 7.1.1, 7.1.2(2019.3.0b4)
Could not test with: 2017.4.35f1 because HDRP is not introduced

Notes:
1. Video of reproduction attached in Edit
2. Not reproducible with the built-in pipeline, URP
3. The workaround is to toggle on lighting or change the Z-axis value of GameObjects
4. Reproducible in Prefab Edit Mode
5. Reproducible on macOS, Windows