Simple pytest(qtbot) QWidget test crashes

I have the following code in test_qtbot.py which shows a dummy widget for one second

import time
from PyQt5.QtWidgets import QPushButton


def test_widget(qtbot):
    widget = QPushButton()
    widget.show()
    qtbot.addWidget(widget)
    now = time.time()
    qtbot.waitUntil(lambda: time.time() > now + 1, timeout=5000)  # show it for one second

This test runs fine when I run it with pytest test_qtbot.py

However, I also want this test in a headless environment hence I tried

Xvfb -ac :12 &
DISPLAY=:12 pytest test_qtbot.py

but this results in a core dump

However, the following does work

Xvfb -ac -screen 0 1280x1024x24 :12 &
DISPLAY=:12 pytest test_qtbot.py

Any idea why the first test fails?

Some additional observations: my test runs on Qt5, python3 and Debian Stretch. A similar test, on Qt4, python2.7 and Debia Wheezy runs just fine without the -screen parameter