Cursor up/down moves to incorrect position after having deleted characters with backspace

If I have two lines of text in a QTextEdit and in the middle of one line I remove some characters with backspace, then moving to the other line using cursor up/down will not move to the position above/below the current one. Instead it will move to the position above/below the one I was before deleting characters.

This is not the behaviour I would expect and I think it is a bug.
What makes it even more confusing is that when you move the cursor back to the original line, it does not go back to the position it was on before.

The problem can be reproduced with qtdemo:
1) start the Main Windows/SDI example
2) type two lines containing "123456789" below eachother
3) position the cursor after 5 on the first line and hit backspace 3 times (removing 345)
4) press cursor down
Actual result: the cursor is between 5 and 6 on the second line
Expected result: the cursor should between 2 and the 3 on the second line
5) press cursor up
Actual result: the cursor is between 8 and 9 on the first line
Expected result: the cursor should be between 2 and 6 on the first line