Simple 2-layer file here. My first foray into Cycles.
The second composite layer has numbers around a big cross, as you see it here, then streak-glared.
Visually, there’s no problem here (that increased sampling can’t cure). But when rendered, the “numbers” composite layer is on black, rather than transparent.
‘Use Environment’ on both composite layers is unchecked.
Any clues? I’d rather have the numbers isolated on alpha, not black.
Because when you use a transparent background, you also have to black it out when viewed directly (camera rays). Otherwise antialiasing creates problems when you try to replace the original sky with a different one.