ID 359154 - Bevelling (shift) a group of polygons with non-uniform vertex normal's yields non-planar results.

Problem summary
Using Polygon Bevel (shift) on a group of polygons with non-uniform vertex normal's yields non-planar results.

Customer reported version
modo.12.1v1

Customer reported platform
10.12

Steps to reproduce

1) Open the attached scene file. It contains a cube with a Vertex Normal map, and 2 vertices of the red polygon have had their normal direction altered.  Ensure you've enabled Vertex Normal visibility under View > Show Vertex Normal.  

2) Select the strip of red polygons.  

3) Enable the Bevel tool (shortcut 'B').

4) In the Bevel tool properties, ensure "Use Vertex Normal Map" is enabled.

5) Activate the tool in the Viewport and drag the bevel upward using the blue tool handle. The Bevel shift will follow the direction of the non-uniform vertices, but the result will be non-planar. 

Workaround
n/a

Reproduced by support
This bug has been reproduced in:
Windows 10
12.1v2
12.0v1

macOS 10.13
12.1v2
12.0v1

Earliest version tested
12.0v1 - The "Use Vertex Normal Map" Bevel tool feature did not exist before this version.

Expected behaviour
The result should be a planar group of polygons, still adhering to the vertex normal direction set by the Vertex Normal Map.

Actual behaviour
Non-planar shift result.