Its posible the golden servent is Lancers master I guess and that lancer was summoned in much the same way as Assassin was but having the supposid end boss have a major battle in the next episode and having lancer disappear for this long just all feels wierd to me.
It does seem like they have been trying to cram alot of content into just 24 episodes, and I believe this series could have easily gone another season. That being said, isn't it also possible that Lancer was already defeated by one of the other servants and they just didn't have time to show it? (though that does seem unlikely considering that anime almost always explains every part of the plot sufficiently)