ID 488557 - Hydra: MergeMat is not working in Hydra 3D view for projections in Nuke

Problem summary
MergeMat is not working in Hydra 3D view for projections in Nuke.

Customer reported version
nuke.13.1v2

Customer reported platform
windows10  

1) In Nuke, create a Checkerboard and ColorWheel node.

2) Add a MergeMat node downstream, connecting the Checkerboard to the B input and ColorWheel to A input.

3) Create a Sphere node and attach the MergeMat to the input.

4) Press Tab in the Viewer to switch to 3D mode, and observe the projection.

Expected behavior
The Checkerboard and ColorWheel should appear merged.

Actual behavior
Only the Checkerboard node appears (whichever is in the B input).

Workaround
Setting the Viewer to Nuke allows the projection to appear correct. (Viewer Properties bin > 3D tab > renderer dropdown set to Nuke)

Reproduced by Support in:
Nuke 13.1v3 - Windows 10, CentOS 7

Nuke 13.1v1 - Windows 10

Nuke 13.0v1 - Windows 10, CentOS 7

Unable to reproduce bug in:

Nuke 12.2v10 - Windows 10, CentOS  7

Earliest version tested
Nuke 12.2v10 - This feature did not exist before this version (Hydra Viewer)