Might be because of the bots? Due to the way I added bots to TDM (and CQ as well, not sure why OBJ mode ends on it's own. Don't recall having bots in that one).
Unlike in proper COOP mode, the server considers them part of the player count (despite the fact they don't show on the player count visible from server browser) and thus the map never enters pre-round. So if there's a round timer set on the server it will always start on those modes even if the server is empty due to the bots still being counted as being counted towards server population.