Drawing a transparent image within the warp leaves artifacts matching the background colour

[BenBergman]

I am trying to draw a png containing an image with feathered transparency.

This is the image I'm trying to draw:

I'm drawing it over a video clip. If I draw it outside of a warp It looks like this:

With the video and image inside the warp I get a weird artifacting around the image that looks like this:

The artifacting colour changes with ofBackground(), but that isn't an issue outside of the warp.

The issue might be related to this but I've run out of time to investigate further today.