When the last survived human buy a survivor and kill zombie he got spawned again (the zombie). I think survivor need to survive not to playing to end of round. I think the survivor should kill the zombies.
it's supposed like that because it's normal infection mode, if u buy survivor on normal infection mode the zombie will respawn, it's already like that from a long time ago and also if u havce money and level why u can't buy the survivor and zombie have limit respawn tho