Combat does not stop once the last boss dies. It is still a problem and one of the most annoying/frustrating bug with new players in the raid (releases and gets stuck outside because an encounter is in progress).
Having the boss spawn right on top of the previous encounter is also annoying, i suggest adding a chest for that one near the entrance or just making the start of the last encounter a dialog option.
I know it has been reported a few times but maybe it wasnt clear enough. Please fix this m’lord devs /pray