I can't say with 100% certainty that anything is "impossible" since there are a lot of variables at play, but I will say that this sounds extremely unlikely.

The fact that it is being reported for one particular process also makes it more questionable.

If they are clicking the Submit button and actually making it to the subsequent page, whether that's the default "Thank You" page or a redirect, there absolutely should be some record of it.

If they are submitting and hitting some kind of error that prevents Forms from generating a new instance, they should see an error page, and you would likely have something in the logs.

In all the years I've been using Forms, every time someone has reported something like this it has turned out to be something else entirely after further investigation.