That is an odd situation, come to think of it, that was something I had thought would be more realistic because then the AI would use the entire runway for takeoff. I had even tried to get my AI to cross 22R to go to Taxiway Z to queue for 31L, and I thought it was impossible! I don't know why its happening on your system, but all my AI use KK.
I'm not sure if in real life traffic uses Z for the entire runway, or if they position at the extended landing threshold like I have it set on my afcad, but if it turns out the Z taxiway is really used in real ops, then I wouldn't worry about it. Does anyone know for certain where takeoffs start on 31L?