Could be a placement issue with the film holder. Could be a shift combined with tilt. Until I got my Deardorff, I never knew you could make shadows on the film plane from the bellows getting in the way via too much shift (lateral and rise and fall). I always shot speed graphics and that was never an issue. That's my bet. Too much rise and fall on one or both standards.
Never the less. Keep working on composition and enjoy your first attempt. I still have mine from 26 years ago.
tim in san jose