Transparent material with group masked material over it rendering incorrectly in RT CPU

[1funk]

Barend discovered this problem on skype, when trying to add a label on a transparent bottle. I have put together a simplified scene and attached it.

LEFT - no label material
MIDDLE - has a label material done using a group mask
RIGHT - has a label material done using blended shaders

You can see RT CPU renders the middle one incorrectly

[BVA]

Thanks Funk! Save me a bit of time.