Youre right, they have faster transition between running and attacking than the player, nice find. i did some testing and found out why that is, somehow they are able to completely stop running and go instantly into combat stance, for a player to be as quick you have to do a turn or something in order to loose the momentum from running so the character can get into the combat stance. so its probably a bug that allows them to cancel out the running mode. its not something thats intended, player and npc's are supposed to play with the same rules so if you find more it would be nice if you report it@Parco
Kinda missed your post, thus the late reply.
The rules are not the same, the AI has much faster transition between running and attacking than the player. The player has to come to a halt from running before getting ready to swing, unlike the AI. You can't strike the AI even if you catch up as you take a second to get ready and the AI runs off again.
There is a lack of proper counters to fleeing enemies currently, this is why some people are not happy with the game. Implementing more realism shouldn't be at the expense of gameplay.
What I posted on steam earlier:
"The issue isn't about realistic AI, it's about killing the fun of the game. Keeping the game fun is critical to game design.
Future skills may provide more counters to fleeing in the future - add fleeing in the future then."