ID 309226 - Alpha Masking Toggle on merge node does not produce results according to tooltip documentation.

Problem summary
Alpha Masking toggle does not represent expected result based on tooltip.

Customer reported version
nuke.n/a

Customer reported platform

Steps to reproduce
1) Open Nuke.
2) Create a Merge Node.
3) Hover over "Alpha Masking" - Tooltip implies that the input image should be unchanged where the other image has no alpha, the images should use the merge operation when both alphas are not 0, and the alpha channel should be set to a+b-a*b.
4) set inputs of merge node to two images with two different alphas.
5) Enabling "Alpha masking" option with any valid operation selected results in unexpected results.

Workaround
Unknown.

Reproduced by support
11.0v2 - Windows 10 - OSX 10.12.5 - Centos 6.9
11.0v1 - Windows 10 - OSX 10.12.5
10.5v5 - Windows 10 - OSX 10.12.5
10.5v1 - Windows 10 - OSX 10.12.5
10.0v6 - Windows 10 - OSX 10.12.5
10.0v1 - Windows 10 - OSX 10.12.5
9.0v9 - Windows 10 - OSX 10.12.5
9.0v1 - Windows 10 - OSX 10.12.5
8.0v7 - Windows 10 - OSX 10.12.5
8.0v1 - Windows 10 - OSX 10.12.5

Earliest version tested
8.0v1

Expected behaviour
Expected The same operation, except where the other image's alpha is 0. The Alpha should adhere to a+b-(a*b). 

Actual behaviour
Operation occurs even where other image's alpha is 0, but not in all cases. Alpha adheres correctly.