They definitely are not. Unless you have people dying. There is a possibility they can spawn prior to getting the mark. And absolutely spawn the moment it fades, but there none that spawned as a wave for us during the mark phase. I believe we had one left up when we went to hallways and two deaths created 2, when we returned we had 8 to deal with. Watched another twitch stream that cleared all before marks and had 0 spawn during mark phase and returned to 5 when they spawned on mark fade.