Spot Render Semi-Transparent Black Overlay

kickflipsonly

I can't seem to find anyone addressing this issue, hopefully I can explain this correctly.

Has anyone noticed when you spot render a particular area within your Viewport, and then overlay it on top of the same background that's a different image, sometimes there's a black, semi-transparent overlay present on the entirety of the spot rendered image, for no apparent reason?

It doesn't happen all the time, which is why it's confusing to me. I've tried various render option settings to troubleshoot it, but haven't had any luck in determining the cause of the black, semi-transparent overlay on spot renders.