setFixedSize(sizeHint()); glitch on dialog move

I created a custom QDialog with a vertical Layout (tabwidget and a horizontal Layout (buttons) below it). Each tab has its own vertical Layout with a Groupbox at the bottom, divided by a vertical stretch from the other widgets above. In order to make the dialog resize correctly on different dpi factors, I use setFixedSize(sizeHint()); at the end of its constructor. This works fine with all my other custom Dialogs. Yet when I open the Dialog in the running application A: some labels are missing the last character and B: when i move the dialog it changes its height on every time I start grabbing. To be precise it switches back and forth between the same two heights. This should not happen on a fixed size. I'm assuming it is related to the stretches?