Well, not sure if it should be considered a bug or if is a game design choice made on purpose, but if I kill all demons from one group except one, then I run away from it into another group of demons, then I Charm the dude from the first group who was following me in order to have him fight the second group of demons on my side, he doesn't do so, is like he doesn't see the other demons because they are not part of his original group.
Though from a charmed enemy, one would expect that he would just protects the summoner from any harm, regardless of small details and formalities like if he knew those other demons beforehand
That's why I've reported it as a bug.