The last time I used extreme tilts, it resulted in a curved line at the top of the photo. This appears to have been the result of not seating the film holder completely into the camera back, since you are positive you pulled the slide fully out, and reinserted it fully.
I second the theory of not seating the film holder all the way. I have had this problem with a Polaroid back because they are thicker. The results were identical...
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.