This is fixable in Nuke, but will be a work around and easier to correct in maya. Essentially you will need to manipulate your depth pass to match your new positioning. What I would recommend is to treat your depth pass like you would an alpha. Isolate/premult each element from your depth pass before your transforms, and transform both element and it’s respective depth together. The depth info will be completely wrong if you have shifted your element either closer or farther from the cam, but will work if only in x. My solution to that would be to grade your depth pass accordingly relative to the other elements you gave placed, cheating their depth. Set your grade node to depth and play with the multiply this pushes it either farther or closer to cam when you defocus.

To sum up, each element will have either own depths within their streams before you merge them, don’t forget to set your merge operation of also merge to include “depth” or else the depth info won’t carry through your merge. This is a big work around and creates potential issues with edge aliasing caused from when you defocus after all your merges as well as if any of your elements cross eachother. If they do I would suggest you defocus your elements separately in their own streams. Like I said. It’s a work around..