ID 405283 - Flat lighting mode in the VRayMtl is incorrectly displaying the transparent checkerboard

Problem summary
Flat lighting mode in the VRayMtl is incorrectly displaying the transparent checkerboard 

Customer reported version
4.5v2

Customer reported platform
Unknown. 

Steps to reproduce

1) Open Mari and the example head project 

2) Go to the Shaders palette and create a new VRayMtl shader

3) Plug in the Diffuse_Head_Geo channel in the Diffuse Color input 

4) Press F1 for setting the lighting on the main mesh to Flat 

result: The transparent checkerboard appears 

Expected behaviour
Mari should display the Flat lighting mode

Actual behaviour
Mari shows the checkerboard:

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Workaround
Use the Current Channel shader 

Reproduced by support
This bug has been reproduced in:

4.5v2 - Windows 10 - CentOS 7 

4.5v1 - Windows 10 - regression

Unable to reproduce bug in:
4.2v2 - Windows 10 - CentOS 7 

Earliest version tested
4.2v2 - This issue doesn't appear in this version and has regressed