The image in question is using an array based image mask also know as Colour Key Masking. It turns out we have no support for this in our rendering core which likely means this is very rarely used part of the spec. In this case /Mask [0 0 ] which would make the white image fully transparent. This will be relatively straight forward to implement.
Traced out the logic for colour key masking and it appeared like everything was in order. The only short cumming as was the buffer didn't actually have an alpha channel.
Added a check for alpha and if not present will copy the the image mask data to 4 component types. This allows the alpha to be painted correctly.