Qt QTreeView Expand Icon change bug

On Windows 7 and later, Qt TreeView expand icon is a small triangle.
but it has change to change back to old style +, - icon. which cannot reproduce on Qt 5.5.1

steps:

1. start Qt Creator 4.0.0 build on Qt 5.6
2. create a new Qt widget project and see the project tree works good.
3. drag a QTreeWidget on QMainWindows and add some items contain subitems
4. File -> New File or Project, and just click cancel

5 All expend icon include Qt Creator Object Inspector and Property Editor expand Icon change to old + - style and when you mouse over, it will become a empty square.

I don't think this is Qt Creator bug, because I build Qt creator using Qt 5.5.1, no this issue.