Welcome Toad! And thanks for sharing your perspective. Absolutely it is complicated and there could be many reasons for this same behavior. I am thinking of really advanced dancers here. Who would be the among the best leads in the room and who have their pick of many followers. Who are confident. (Or at least appear confident, but who knows) I dont know how long you have been dancing tango of course. But it is totally understandable for a newer lead to chose to dance only the last song of a tanda. I don't think anyone would find fault with that. Starting tango is hard enough without adding even more stress of this kind. One particular event pops into my mind. I was at a milonga and a friend arrived quite late and we danced one song of the final tanda together. It was either this one song or nothing. In this situation I was pleased that he made this effort to dance with me at all. It really depends on the situation and the relationship you have with the leader who offers the final song invite.