Update: ok, that was pretty stupid of me :) that "plane covering a window" is actually a light object, right? So it becomes visible when you enable DOF? And that light is against a shadowcatcher material?
Yes, Correct!
......
Apologies that my previous description was unclear! Appreciate the reply nonetheless.
Yes, the furniture is all 3d, the rest is a real photo. I modelled the room to match the photo and applied shadowcatcher to ceiling/floor/walls.
Yes, there is a rectangle light covering the door/window in far center of the image. The 'plane' visible in the first image is the light, which has 'visible directly' unticked. That's why it is not visible in the second image from my initial post (the render which has no DOF applied to the same corona cam). The apparent bug is that the light is visible (albeit oddly blurry) when I apply DOF to the camera.
Does this make sense now?
Any suggestions why this is happening, or how I can have the light turned on, invisible, and also have DOF applied to the corona cam?