I attached a screenshot with Qt Creator 5 beta1 built with Qt 5.15.2 which uses the "windowsvista" style, and also the Qt Creator 5 rc1 built with Qt 6.2.0 which uses the "fusion" style.

With the Qt5 build of Qt Creator I could pass the "-style windows", "-style windowsvista" and "-style fusion" and see the UI changing accordingly.

With the Qt6 build of Qt Creator this doesn't happen. "fusion" is always used.

Note that if I use qtc-askpass from the Qt6 build of Qt Creator the "-style" command line argument works, so this is not a Qt6 problem.