ID 410647 - Purge unused materials purges materials incorrectly

Problem summary
Purge unused materials purges materials incorrectly.

Customer reported version
modo.13.1v1

Customer reported platform
windows10

Steps to reproduce

1. Open Modo.

2. Hold "Ctrl" and click the sphere tool to create a unit sphere.

3. Press "Ctrl + N" to create a new scene.

4. Hold "Ctrl" and click the cube tool to create a unit cube.]

5. In the item list, drag the mesh from the new scene to the original scene.

6. In the import dialog, uncheck all options and click "OK"

7. Right-click the imported mesh and click "Create item mask."

8. Select the imported mesh and press "m" to reveal the polygon set material window.

9. Name the material a generic name and select "OK."

10. Select the Shading tab to see the material created in step 9 nested beneath the item mask created in step 7.

11. Select Texture > Purge unused materials.

Result: The material created in step 9 will be deleted.

Expected behaviour
Used materials shouldn't be purged.

Actual behaviour
Materials being used by the imported object are being purged.

Workaround
Unknown.

Reproduced by support

This bug has been reproduced in:
13.1v1 - Windows 10 - MacOS 10.13 - Centos 7

Unable to reproduce bug in:

13.0v1 - Windows 10 - MacOS 10.13 - Centos 7 -regression

Earliest version tested

13.0v1 - This issue doesn't appear in this version and has regressed.