ID 404114 - Semi-transparent pixels get darker when a filter has been applied

Problem summary
Semi-transparent pixels get darker when a filter has been applied on the paint buffer. 

This is affecting all Filters. 

Customer reported version

4.5v2

Customer reported platform
windows10

Steps to reproduce

1) Open Mari and the Example Project 

2) Select red colour and paint on the model without baking 

3) Go to Filters>Gamma and make sure that Apply to is set to Current Paining 

4) Adjust the gamma value and Apply the filter

result: Semi-transparent pixels get darker than expected

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Expected behaviour
After applying the filter, the paint should look the same as the preview 

Actual behaviour
Semi transparent pixels get darker when the filter has been applied to the paint buffer

Workaround
Unknown.

Reproduced by support
This bug has been reproduced in:

4.5v2 - Windows 10 - CentOS 7 - regression

Unable to reproduce bug in:
4.5v1 - Windows 10 - CentOS 7 

4.2v2 - Windows 10 

Earliest version tested
4.5v1 - This issue no longer appears in this version and has regressed