The AI surrender behavior was introduced a few updates ago. I believe a successive update made the AI's less prone to surrendering on a hair-trigger. If you don't have the most recent version, that could be your problem right there. I don't have entrenchment, but I understand that the surrender behavior is the same as it is in the most recent version of vanilla.
One of the underlying problems was that some players would just build up fleets up to the 2000 fleet limit, and only then attack. The AI, which has had a smaller economy all game, is in a hopeless position where it's greatly outnumbered by more technologically advanced units. The game has been a forgone conclusion for hours, and you've only consolidated your advantage in the meantime. The AI is unlikely to kill anything, much less actually win a battle, so it surrenders.
In my experience playing vanilla, the AI rarely surrenders, but when it does it's often unpredictable. Usually I have to reduce the enemy's fleet to ashes and be bombing its homeworld before it surrenders, so in my experience this isn't a huge problem. Sometimes I do get a surrender early, but it's rarely before I've toasted their fleet and am bee-lining for their homeworld.