When you have a text that ends with a diacritic (https://en.wikipedia.org/wiki/Arabic_diacritics, https://en.wikipedia.org/wiki/Hebrew_diacritics), pressing backspace will ignore it and remove the whole character (what is called cluster).
In normal text edits (QLineEdit, QTextEdit), each diacritic is removed first, and when there is non left, another backspace press will delete the character.

The reason of this is using "previousCursorPosition" method, as cursor doesn't move when adding diacritics. And so, the previous position will be the last character.

Sample text (With diacritic at the end): "ذهبَ"
Sample text (With two diacritics at the end): "حيٌّ"

Reproducible: Always

Steps to Reproduce:
1. Write/Copy-Paste sample text
2. Press backspace

Actual Results:  
The character with it's diacritics are all removed

Expected Results:  
َ    diacritic is removed in the first sample text, and
ٌ    diacritic is removed in the second sample text