Looks like it may be a bug in Qt Creator.
From the QTreeWidgetItem::flags() docs:
If the item was constructed with a parent, flags will in addition contain Qt::ItemIsDropEnabled.
In the generated ui.h file the items are added with the treeWidget as parent and the items are definitely drop enabled, yet the property editor has DropEnabled unchecked.
If you were adding the items in your code you could unset the flag before you add the item to the treeWidget. When you add the items within Qt Creator the only way that I see to unset the flag is to iterate through the items.
Try adding something like the following to your treeWidget constructor:
for(int i=0; i<ui->treeWidget->topLevelItemCount(); ++i)
ui->treeWidget->topLevelItem(i)->setFlags(ui->treeWidget->topLevelItem(i)->flags() & ~Qt::ItemIsDropEnabled);
for(int i=0; i<ui->treeWidget->topLevelItemCount(); ++i)
ui->treeWidget->topLevelItem(i)->setFlags(ui->treeWidget->topLevelItem(i)->flags() & ~Qt::ItemIsDropEnabled);
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