It is a regression from 6.5.3 to 6.5.5, and affecting all latest patch versions including 6.5.10, 6.8.4 and 6.9.2.

The reproducer is a a QMainWindow showing a trivial subclass of QMessageBox. By "trivial", the subclass is really not doing anything special and everything should be the same as a default QMessageBox.

And it works as expected back in 6.5.3:

But since 6.5.5, the message box looks like "old-school desktop popup":

Using QMessageBox as it is is of course alright. But subclass of any kind is a problem.