yeah sometimes it can be annoying when they retreating so far back leaving you behind. But sometimes its also really tactically good since they utilize their archers and your reinforcements when enemy AI overextend...But yeah they have to do an rework or fix with AI command in battle even if AI have saved me from really tricky situations at times ^^
Indeed, AI will make tactical retreat when reinforcements are coming. It also is worth paying attention to overall commander, when you are fighting as a commander of larger army. He will generally tell what AI is up to. His messages are easy to miss in the heat of the battle and you can easily get yourself isolated with your group of archers when AI retreats to regroup.