Sample project at https://github.com/nyanpasu64/qt6-dpi-fail/tree/qlistwidget. The core of the code:

auto w = new QListWidget;
w->setDragDropMode(QAbstractItemView::InternalMove);
w->setDragDropOverwriteMode(true);
w->addItem("hello");
w->addItem("world");

When I click the "hello" item, then drag it half an item down to release it on the upper half of the "world" item below it, then an empty item is spawned above the "hello" item and remains selected. This can be repeated to spawn many empty items in the QListWidget.

Video attached. (Ignore the mouse cursor glitching up, this resulted from logging in at 125% DPI and switching to 100% DPI, then recording in ShareX. The bug occurs at 100% DPI as well.)