Windows as tabs not supported in Qt 5.15

On previous Qt versions, multiple windows are shown in tabs instead of separate windows if

a) the user is in full screen mode, or
b) the user has selected 'always' for the 'prefer tabs when opening documents' option in the system preferences>dock section

In Qt 5.15.0, in both cases two separate windows are shown instead.

The following PyQt app demonstrates the problem:
 

import sys
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *
from PyQt5.QtGui import *

app = QApplication(sys.argv)
d1 = QDialog()
d1.show()
d2 = QDialog()
d2.show()
sys.exit(app.exec_())