Button style is incorrect for an inactive window in MacOS

Hello,

There seems to be a bug which occurs when opening an inactive window on MacOS. If a window is opened in the inactive state, the button with appear with the wrong style and no text.

This behavior can be seen in the attached example as well as the screen shots. To run the example simply paste the main.cpp in a basic widgets application and run it. A blanc window will appear, if you deactivate the window, by moving to a different application, it will cause and inactive window to appear. This is where you can see the bug in the buttons appearance. 

This bug is not present in Qt 6 as I have also shown in with the screen shots, but I believe it should be fixed in Qt 5.15 as well.