Create a quiz.
Add three questions.
Within the question overview page customise the question numbers as follows
Question 1 => 1.1a
Question 2 => Must fix 4.2
Question 3 remains unchanged
Preview the quiz.
Verify that the very long numbers don't break the display.
Steps to reproduce
Create a quiz with, at least, one question.
Set the number to something long (For instance, 1234567890123456).
Preview the quiz (or attempt it as a student).
Check the customised question number is displayed out of the box on the left of the question and also in the Quiz navigation block: