Using the default cube scene, I’ve enabled the Workbench renderer & also “Film → Transparent”.
When rendering my cube, besides the background being transparent, which is what I’m expecting, the cube itself is 87.5% transparent. I’ve tried with many different scenes, and this is always the behavior, down to the same level of transparency on what should be solid pixels.
Is this a bug, or am I misunderstanding something?
It looks like a bug to me, what you can do, for now, is to set your world background color to bright green, turn off transparent and render. Then you can knock out the background with a green screen tool.
I just reported it, but you can comment that you experience the bug, too.
You have to sign up to participate, however. https://developer.blender.org/T65914