Oh.. I figured it out.
That's just a simple unit conversion error.

I used millimeter instead of pixel...

>>> # Python >>> __editor.mmMarginLeft() 20 >>> __editor.pxMarginLeft() 110 >>> __editor.mmMarginLeft() == __editor.pxMarginLeft() False

Now it's solved! THANKS a lot!
Screenshot 2022-11-27 at 4.29.45 PM.png

@tapsbrighton This is a better solution rather than append:

insertPlainText("\n");

Terribly sorry, but it turned out to be my fault all along.

The line responsible was:

textDoc.setDocumentMargin(0);

in the sizeHint() method.