ID 369193 - Constant layers in material groups cause meshes to appear black in the Advanced Viewport

Problem summary
Constant layers in material groups cause meshes to appear black in the Advanced Viewport. 

Customer reported version
Modo 12.1v1 

Customer reported platform
Windows 10 

Steps to reproduce

1) Open the Viewport Settings by pressing "O" and go to Advanced Settings. Enable "Use Normal Maps" and "Use Bump Maps". 

2) Change the Viewport display mode to "Advanced". 

3) Create any type of mesh. 

4) Select the mesh and assign a material by pressing "M". Give the material a name and select type as "Unity Material".

5) Go to the Shading tab. Setup the following structure (see attached image) for the Unity material: 

• A Unity Bump/Normal texture map.
• A Constant layer and set effect as Unity Albedo. 
• The already existing Unity material. 

6) The mesh should now incorrectly display as black in the viewport. Please see the attached image for an example of the issue.  

Workaround

Unknown workaround. 

Reproduced by support

This bug has been reproduced in:

Modo 12.1v2 - Windows 10 - CentOS7 - MacOSX 11.13 
Modo 12.0v1 - Windows 10 - CentOS7 - MacOSX 11.13 - regression 

Unable to reproduce bug in:
Modo 11.2v3 - Windows 10 - CentOS7 - MacOSX 11.13 

Earliest version tested
Modo 11.v3 - Issue no longer appears in this version and has regressed. 

Expected behaviour
A Constant layer should change the base material's colour while retaining the bump/normal map shading. 

Actual behaviour
Having a Constant layer in a material group causes the mesh to appear black.