Raytraced with transparent background - ViewCapture background black?

I made a copy of Raytraced and set the background to transparent. ViewCaptureToFile with the transparent background box checked shows the background as transparent in the preview, but the saved .png has a black background. Am I missing some setting here?

Hmm, never mind, after trying some different settings, it seems to have fixed itself - but I have no idea what actually fixed it… :confused: