They do reset by the time a new team/player enter the boss room. I mean, there is a second where the new team can see the floor that the last team left behind (dead bodies, loot, etc), but they won't be able to take or check anything cause by the time they enter it resets.
It has happened to me several times that when I enter falcon boss, I can see his dead body for a second, and then the room resets.