Designer: Morphing a widget messes up its order in QSplitter

I had a QMainWindow, with a vertical QSplitter in its central widget. It had an empty QGroupBox at the top, and a QTabWidget at the bottom.

I right-clicked the QGroupBox and selected "Morph into" -> "QStackedWidget". It became a QStackedWidget, but it jumped to the bottom half of the QSplitter (I expected it to remain in place)

I then undid the action by pressing Ctrl+Z. The QStackedWidget turned back into a QGroupBox, but it STILL remained at the bottom (I expected it to return to its original position)