(sidenote: this setup is a little bit quirky, sometimes during waking up one of the displays wakes up as 1080p (or 4K@30Hz), requires turning it off/on and switching resolution. Still, none of these should cause crashes and assertions). 

The problem seems to be new code in qcocoascreen.mm; The callback registered through CGDisplayRegisterReconfigurationCallback is fired with events in order Qt can't handle properly. 

Also Qt makes assumptions that seems not to be true:

For example the callback is invoked with displayId == 4128830 and flags == 4368 (kCGDisplayAddFlag, kCGDisplayEnabledFlag, kCGDisplayDesktopShapeChangedFlag), so Qt assumes CGDisplayCreateUUIDFromDisplayID will return valid value (to match displayId with NSScreen), but in my case CGDisplayCreateUUIDFromDisplayID(4128830) returns null despite callback being called with added/enabled flags. This causes assertion in QCocoaScreen::updateProperties()

Also note that in this situation, there are two [NSScreen screens] present, but neither deviceDescription->NSScreenNumber matches the displayId; 

Edit: Attached a workaround patch I'm testing right now that seems to solve the assertions and crashes in this case. While not a proper solution, It still might be helpful.