I've been loading film reels with leaders attached into the content window and then trimming off the heads and tails. When I go to the timing pane and say to trim 8 seconds off the head, and then go to the first frame in the preview window, I see the frame from exactly 2 seconds before (in this case the 2-pop frame). I thought this might just be a problem with the preview window, but this extra/replaced frame did also render into the DCP.
I checked the changeover from reel 1 to reel 2 and beyond and everything from 2 onwards was fine, so it seems to only affect the first piece of content and not the ones after it.