Wendy,
Is there any chance the students were taking the exam while in the proximity of a black hole?
What Happens To Time Near A Black Hole? - YouTube
Sorry, obviously I'm kidding and I hope you found humor in that response and smiled at least a little. We all could use smiles right now. 
I don't know how this could have happened and it's really a bit difficult to guess without seeing actual logs. I know I have seen logs of students who appear to have had longer times than they should have on a quiz by a few minutes. Usually upon closer inspection they are not actually working on the quiz questions longer, it just appears to be a delay on submission which I've attributed to internet congestion. It's only been once or twice, but my main point is that maybe something else can explain what you are seeing other than time falling back for select students.
You did mention lockdown browser so I wonder if you are also using Monitor with that and have video of them taking the test which hints at time differences. If so, the issue could be related to the video and frames per second recorded. Most video consists of repeated frames of 30 or 24 frames being captured and/or displayed per second. If their computer couldn't keep up with capturing video at that same rate the entire time, that also could explain a perceived time drift. Video starts out being recorded at 30 frames per second, but as their computer gets more taxed by activity it slows down to capturing 28, then 26 and maybe even down to 20 frames per second. But the system still thinks it's 30 frames for second video and thus time may not be accurate.
Those are just some thought Wendy and really are guesses. I don't have a good explanation but will suggest that contacting Canvas support to take a look at the specifics such as logs from the quiz might provide some better insight.
Rick
