I was running several mix windows as keystone windows in order for them to be transparent as they overlap in places.

This all went well.

I then started playing with blend types and found blend C to work really well with this setup. Unfortunately this led to any area where 2 mixes overlapped ceasing to be transparent.The upper mix became black background wherever it overlapped another mix with blend C. Change the blend back to A or B on either mix fixed the problem.

This was using build V4.4 m364