Thank you for your report! I will add this to our bug tracker.
If you need a workaround, I think you could swap the red and blue channels before encoding:img.RenameChannel( 'R', 'temp' )
img.RenameChannel( 'B', 'R' )
img.RenameChannel( 'temp', 'B' )
enc.EncodeNextFrame( img )
(This requires Draft 1.1.)