Right now I am playing a 0 turn campaign with the 2.6 version. I had a full a legion with polybian legionnaires with one stone thrower unit, one archers, one slingers, one velites unit, three cavalry units including the general and the rest are the polybian cohorts with four units of triarii.
I attacked a Greek army near a city called Philipoupolis and being near the settlement its garrison came for the help. The battle map consisted of a kind shallow uphill battle with some trees to the left and from the centre to the right it was an open field. I put my stone throwing ballistas behind the first line of infantry until the enemy, joined together as a one full army, came within the striking distance of my maniples.
To come to the point, some of the ai cavalry surprised me from the behind of the left flank and was caught unaware because I was fixed on the the centre and left flank to flank out the enemy. Their cavarly attack the velites and archers that were in between the second and first lines while some of them charged and the back of the already engaged infantry.
I wanted to post because I was never surprised by the AI like that. They are tending exploit any form of gaps if you are not careful enough.