The same thing happened in Rome 1, the best way to attack with pikemen was to order them to move to a position behind the enemy line. I'll give the tactic a ago in Rome 2 once the Seleucid DLC is out.Units still lose all cohesion and formation when attacking tho - gives a huge bonus to the defender who can hide behind the phalanx.
Yes I am aware of this but this tactic is risky since you never know if your units wont just try to move through the enemy forces. AI tries force moving and we see how that works.
Also remembered another quirk of the AI - if it gets reinforcements it tries to combine its forces as soon as possible, often by force marching troops ignoring the opposition it encounters.