Hm, I really feel the Opening Game is generally too long. I almost always have to trigger it manually because the AI will just sit there waiting for nothing apparent. I'm sure it would have launched the middle game eventually, but I feel it waits so long in the setup phase that it runs out of time in the end game if it meets resistance. Also, I feel it should maybe more agressively pursue the objectives, at least in the end game. Or then we need a surrender mechanic.
Also, if I manually trigger 'Middle Game' in the AI window, already twice now it triggered 'End Game'. I thought it was a statistical fluke but it might be a bug.
I did a deep offensive recon and watched as the AI came back to assist its rearguard units fend off my attack, so that works! Very cool!