ID 447161 - The "all" option does not appear in the FrameBlend's channel knob dropdown menu

Problem summary
The "all" option does not appear in the FrameBlend's channel knob dropdown menu.

The default value is "all", however, once you change the knob value, you can't select "all" from the dropdown menu again.

Customer reported version
Nuke 12.2v2

Customer reported platform
CentOS 7

Steps to reproduce

1) Open Nuke.

2) Create a FrameBlend node

3) Change the channel knob from "all" to any other option.

4) Try to change the knob value back to all, and notice that the "all" option no longer appears in the list. 

Expected behaviour
The "all" option should appear in the FrameBlend's channel knob dropdown menu.

Actual behaviour
The "all" option does not appear in the FrameBlend's channel knob dropdown menu. Once you change the knob value, you can't select "all" from the dropdown menu again.

Workaround

If you want to change the knob value back to "all", you can do one of the following:

1) Right click the Properties panel and select "revert knobs". However, this will revert all knob values on that node, so if you have made any other changes, these will also be reverted.

2) Alternatively, you can select the node in the Node Graph, then run the following line of code in the Script Editor:

nuke.selectedNode()['channels'].setValue('all')

Reproduced by support

This bug has been reproduced in:
Nuke 12.2v2 - Windows 10 - macOS 10.14 (Mojave) - Linux CentOS 7

Nuke 12.0v1 - Windows 10

Nuke 11.0v1 - Windows 10

Nuke 10.0v1 - Windows 10

Nuke 9.0v1 - Windows 10

Nuke 8.0v1 - Windows 10 - macOS 10.14 (Mojave) 

Earliest version tested
Nuke 8.0v1
- This issue appears to be in all versions of the product