Bug IN-19680 - Shader: VFACE Regression in Unity 2020/2021 (compared to 2019)

Update: Unity QA was able to reproduce the problem and it can be found in the public issue tracker now: https://issuetracker.unity3d.com/is...ertically-when-mirror-horizontally-is-enabled

Hello @richardkettlewell , perhaps you have an idea why it doesn't work in 2020 and newer anymore.

Using VFACE in a shader behaves differently between Unity 2019 and 2020. The "broken behavior" seems to affect only meshes that are rendered via a ParticleSystem.

Please see the attached video. I explain in detail what doesn't work and show how to reproduce the issue.

Reproduce

1. Open attached project
2. Open SampleScene
3. Press Play
4. Select "Main Camera" in Hierarchy
5. Toggle "Mirror Horizontally" in Inspector
6. Notice the Viewport gets flipped horizontally, but the object colors are consistent (they don't change)
7. Repeat the same steps with Unity 2020 or Unity 2021

Actual
In Unity 2020.3.38f1 and 2021.3.11f1 the object colors change, indicating VFACE isn't properly working.

Expected
Same behavior as in Unity 2019. Object colors don't change when flipping the viewport.