ID 387321 - An EXRs Write node's first_part knob does not work when write_full_layer_names knob is enabled

Problem summary
An EXRs Write node's first_part knob does not work when write_full_layer_names knob is enabled.

Instead the first channel will be the channel at the end of the first_part knob enumeration list.

Customer reported version
nuke.11.3v2

Customer reported platform
centos7

Steps to reproduce

1) Open the attached Nuke script

2) Render the two Write nodes

3) Read them back into Nuke, and view the metadata. With write_full_layer_names disabled, the exr/channels shows the motion channels, with is enabled, it shows layer1.

Expected behaviour
For the first channels written out to be U and V or the motion channel

Actual behaviour
The first channels written out were A, B, G and R of the layer channel

Reproduced by support
This bug has been reproduced in:

Nuke 11.3v3 - Windows 10, CentOS 6.9 - MacOSX 10.13

Nuke 11.2v3 - Windows 10, CentOS 6.9 - MacOSX 10.13

Earliest version tested
Nuke 11.2v3 - This feature did not exist before this version