The bug was independent of it being the first round or not: if a player joins in the middle of any round, it is impossible for them to take part in that round, even in the case of a total party wipeout. Of course, a wipeout in deathmatch survivor just means the end of the round, but in these modes it allows play to continue, bringing everyone back from the affected team. Notably, if a player leaves mid-round so that their spot is replaced by a bot in "Waiting" status, the wipeout does allow that new bot to take the field at that time, but the human player is still locked out! That's the big problem with those survivor combinations.
Under the newest revision, freshly compiled and in offline play, I can confirm that the current behavior is that a maximum of one bot per team will spawn in survivor matches of any kind, no matter when the player enters, as though the game is treating it like the other "duke" mutator.