We're investigating Draft Coach right now - can I double check that, though I've looked at both the initial similarity check  & the full report, that it's not including any previously submitted work?
On the full report, it looks as if it is looking at previously submitted work, though the report itself doesn't appear to be picking any up (I know that this paper usually does, as it's one I use for demos, so have to remove the 100% match all the time!) I've uploaded 3 screenshots of the Initial Check, the full report & finally the repositories its claiming to match, included the submitted works. In Feedback studio, this report gives me 100% against itself, and also a few % against other student work)