ID 367332 - Vertex normals behind visible geometry are not always culled even with the "Cull Hidden Normals" enabled

Problem summary
Vertex normals behind visible geometry are not always culled even with the "Cull Hiden Normals" enabled. 

Customer reported version
Modo 12.1v2

Customer reported platform
Windows 10 

Steps to reproduce

1) Open the attached scene in Modo.

2) Go to View -> Show Vertex Normals. 

3) Make sure that "Cull Hidden Normals" is enabled in the Tool Properties. 

4) In the Lists subtab, select the Vertex Normal Map.

5) Rotate the viewport and look at the skull mesh item from above.

6) Normals are not culled as shown in the image below: 

Workaround
Unknown.

Reproduced by support

This bug has been reproduced in:
Modo 12.1v2 - Windows 10 - CentOS 7 - MacOSX 10.13
Modo 12.0v1 - Windows 10 
Modo 11.2v3 - Windows 10 
Modo 11.1v1 - Windows 10
Modo 11.0v3 - Windows 10
Modo 10.2v4 - Windows 10 - CentOS 7 - MacOSX 10.13

Earliest version tested
Modo 10.2v4 - This issue appears to be in all versions of the product. 

Expected behaviour
All vertex normals that are behind visible geometry should be culled.

Actual behaviour
Only some vertex normals are being culled behind visible geometry.