This was during a Gateway Quiz?
It would be worth checking the problems to see if any of them use problemRandomize.pl or PeriodicRandomization.pl, which are designed to produce new versions of the problem. They might not be functioning well in a Gateway Quiz.
If it were a regular set, then I have had the same assertions from students in the past. But I am convinced that they did not hit Submit. Rather, they hit the enter key. Normally this induces a push of the Preview button. But if problemRandomize.pl is used, a "Give me a new version" button precedes the Preview button, and their pushing of enter induces a push of this button. They are convinced they pushed submit, but in those cases I am convinced that it is actually user error.
This shouldn't apply to a Gateway Quiz, because I would hope that the "Give me a new version" button is not provided. But it's still worth investigating if either of those macro files are in use.