ID 444371 - mriCurveLookup returns incorrect values

Problem summary
mriCurveLookup returns incorrect values 

Customer reported version
mari.4.6v4

Customer reported platform

Steps to reproduce

1) Download the attached .xml and open Mari's example head project

2) In the Python Console, run the following code:

mari.gl_render.registerCustomProceduralLayerFromXMLFile("test_Curve","<path/to>/test_lookup.xml")

3) In the Node Graph, create and view a test_Curve node

4) Colour pick the object, it won't be completely black, usually values around 0.013 < This should be 0

5) In the test_Curve node's properties, open the Map attributes Curve Editor

6) Right click>Presets>Linear

7) Colour picking again now returns 0

Expected behaviour
For the correct value to be returned from the mriCurveLookup(), so pure black is colour picked

Actual behaviour
Incorrect values are returned

Workaround
Unknown.

Reproduced by support

This bug has been reproduced in:
Mari 4.6v4 - Windows 10, CentOS 7

Mari 4.6v1 - Windows 10,

Mari 4.0v1 - Windows 10,

Mari 3.0v1 - Windows 10, CentOS 7

Earliest version tested
Mari 3.0v1 - This issue appears to be in all versions of the product