Symfony version(s) affected: 4.3.3

Description
Due to usage of substr in place of mb_substr in backspace portion of QuestionHelper::autocomplete it remove only one byte from the end of utf-8 strings instead of two, resulting in mangled input and possible errors.

Maybe there are reasons not to use mb_substr?

How to reproduce

1. Input any utf-8 string in autocomplete input
2. Tap backspace
3. Now your input string is damaged beyond recognition

Possible Solution
Quick fix that helped me was to replace substr with mb_substr in backspace handling portion of QuestionHelper::autocomplete