Incorrect Results from rev-parse HEAD?

Since it is possible to hack the commit time/date stamp, I have been using the strategy of asking my students to run “git rev-parse HEAD”, and cut and paste the resulting commit hash code in our Learning Management System which has secure time/date stamping. This has worked for me for several years, but this week, all but one of my students cut and pasted the result of git rev-parse HEAD into our LMS, and the resulting hash code LOOKS valid, but does not match ANY of the student’s commits, as seen on the “Commits” tab of the web browser (even if you look at the entire string… not just the part that’s visible.) What are we doing wrong?