Steps to reproduce:
1. Download Repro.zip and open "test" scene
2. Disable the Blast and enable the Ribbon gameobject
3. Press play notice that top line has gradient
4. Go into Player Settings and type BUG_SETUVS in the Scripting Define Symbols field. (This switches the UV creation from Mesh.uv to Mesh.SetUVs)
5. Press play, notice top line now green.

Mesh.uv and Mesh.SetUVs switch is done in PopcornFX/Scripts/Runtime/PKFxManager.cs at line 1594.

Expected results: Mesh.uv and mesh.SetUV should produce same results.

Actual results: It seems like UVs created with Mesh.SetUV won't read back the UV component in the vertex buffer.

Reproduced with: 5.6.0b4, 5.5.0p4. 5.5.0f3