For me, early Hangouts would frequently get stuck in one of a variety of broken states which made it apparent that a “meeting” (the eponymous hangout) was a separate thing in the system with its own identity rather than just a tuple of participants, even for two-party calls. That doesn’t excuse the UI changes (which I guess were done to differentiate the now-business-oriented Hangouts^WMeet from the new, consumer-oriented Duo?..), but the underlying machinery seems to have worked this way from the very beginning.
