Title bar can't be made transparent anymore in macOS on Qt version 6.8.0

I could make window title bar transparent in macOS on Qt versions earlier than 6.8.0 using a mm file. I attached a simple project to demonstrate this bug that is initialized in Qt 6.8.0.
You can build and run the sample project TransparentTitleBar.zip in both Qt 6.7.3 and 6.8.0, and see the difference of title bars.
On Qt 6.7.3, the title bar seems transparent as like below.

On Qt 6.8.0, the title bar can't be shown as transparent as unlike it is on Qt 6.7.3 as like below.