Created attachment 159110 [details]
Recording of the bug.

SUMMARY
Hitting the return key may delete text of remaining line after the cursor if there is a tab character.

STEPS TO REPRODUCE
1. Create a new document with the following content: "\tABC\n\tDEF"
2. Move the cursor before at the beginning of the second line, right before the tab character.
3. Hit <Backspace> and then <Return>. 

OBSERVED RESULT
The text "DEF" vanished.

EXPECTED RESULT
<Return> should reverse the effect of <Backspace>. "DEF" should still be there.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.27.4
KDE Frameworks Version: 5.105.0
Qt Version: 5.15.9

ADDITIONAL INFORMATION
You can find a recording of the behavior in the attachments.