When a student responds to a numeric question with a fraction using the backslash key / (eg, 3/10), Canvas removes the backslash (eg, 310) when the student submits. Fraction answers aren't supported for numeric questions in standard Quizzes either (only for Essay question types), but students will see that the backslash was removed before submitting the quiz.
In Quizzes.Next, students don't know that the backslash isn't supported and can submit an answer that displays as a fraction until after the quiz is submitted and they see their responses.
I submitted a ticket for this issue since it seems to be a bug with Quizzes.Next, but wanted to bring it up here, too, in case anyone else comes across it.
Update: As of 2/8/19, Canvas will give students the following warning message if they try to submit a quiz and have included special characters in a numerical question answer:
This does not affect answers containing a "-" to signify a negative number.