All QtQuick Windows flickers, when opening a second Window

After trying to upgrade our existing QtQuick desktop application to the latest version Qt 5.12.2, everything seems fine, except on macOS, where the whole application flickers when another QtQuick Window was opened.

I tried to reproduce it in a small example application, see attachment.

The example consists of a window containing a button and another window. By clicking on the button the second window will become visible and at the same time both windows flickers with a white color for short time. First I tried a Rectangle component with a MouseArea and after clicking on it the windows don't flicker immediately, but flickered after I resized the main window. After trying to simplify the code, I used a Button from QtQuick.Controls and it started flickering immediately after clicking on it.

Has someone experienced it or knows a workaround?

Thanks for your help.