🐞 bug report

Is this a regression?

No. This behavior worked in the previous version

Description

If the cursor is positioned before the prefix, then entering an invalid character will duplicate the prefix value

🔬 Minimal Reproduction

the problem is reproduced in your demo. I checked for version 11.1.5 and in my project version 8.2.0, also I reproduced this issue on stackblits with ngx-mask version 8.1.6

steps to reproduce:

1. enter some characters
2. highlight value and try deleting using backspace the cursor will be before the prefix
3. enter any invalid character (not numbers), for example the letter "f" numbers from the prefix will be duplicated

Angular Version: 8 - 11