I just downloaded a fresh copy of the file from the prep course, opened it in 2017 SP5 and got the 3 invalid faces, I did not run the import diagnostic when prompted.
I also get 3 faces in SW 2018 SP4.0 and 2019 Sp0.0 so I'm not sure what would be different. Maybe try download the file again and re-open?
I have no idea what happened. It worked on my personal computer using 2017, but not on my work computer using 2018. Maybe it has something to do with my work settings.