This isn't considered a bug.
Companions aren't raiding troops with incentives to "fight them 'all" but they fight when considered necessary or when given reasonable order to attack this or that single enemy.
Moreover, in the heat of the combat, or after, they may take turn or two to consider things - what to do next. Much like a player character also does once in a while.
So, they are also not berserks who would always either fight actively or be in a pursue for the next active fight.