How to reproduce bug:

1. Create a new document in Google Docs with multiple lines, for example with this content :

   first line
   second line

2. Create Qt Widgets Project with following code; build and run it:
   main.cpp

   #include <QApplication>
   #include <QTextEdit>
   
   int main(int argc, char *argv[])
   {
       QApplication a(argc, argv);
   
       QTextEdit te;
       te.show();
   
       return a.exec();
   }
   

3. Copy both lines from the document and insert it into QTextEdit
4. You can see now that the second line overrides the first line
   

This happens only if the selected text contains the end of line and is inserted at the first position of QTextEdit. It seems that the problem is in the line spacing: I was able to move one line from another by pressing the Enter many times.