No focus on window after showing and hiding a modal dialog

For our application we have a modal dialog which is shown while opening a file. When we are done reading the file we hide the dialog and create a new window to show the content of the file.

This used to work in 5.6.0, but after upgrading to 5.12.6, the newly created window doesn't get the focus any more (it's not the keyWindow in cocoa language). This issue still exists in 5.15.0.

In attachment, you can find a minimal example. (although it doesn't reproduce the issue always, 9 out of 10 times the problem is visible)
After some research I found this issue was introduced by https://codereview.qt-project.org/c/qt/qtbase/+/182773
The currentModalSession() call was performing more checks than just checking whether cocoaModalSessionStack is empty.