Ideas
We had this happening to us on Friday where we placed 3rd and spectated the final 1v1, but there was nobody else but the guys that killed us. They didn't win and there was no one else to be seen.
Yesterday(Sunday) we had this happen in quads to us as you described, it has something to do with redeployments and redeployments closing. We had that "Feed Loading 5... 4... 3... 2... 1..." can't remember what it actually says but anyway, then just a screen in flames where we died, couldn't move or do anything except open map to look at the zone. We decided to wait to see if this is what happened on friday and it was exactly same thing, there was 2 teams left and the team that placed 2nd burned to death and we got 1st place.
Also there apparently is a bug where you can glitch vehicles under the map and survive there? Don't know more about it just read in reddit that tanks apparently can get under the map and just stay there till the game ends?