that is not a bug it is a design flaw that is present on any server.
long story short the the bees your Sera summons are pretty much normal mobs, they have nothing to do with you and you dont really have control over them.
if they score a kill on their own then that is not attributed to you, nor you will get loot for it.
because of that adding points to the legion over other skills is ill advised. you should focus on adding Needle of Paralysis and Poison Mist. if your sera has enough damage output to out-damage the summons then you will be awarded for the kill. (dont have a Sera on this server, i am not sure if tagging them is enough, but if you or Sera deals more that 50% of the damage then the kill is yours for sure)
as for the legions, the skills is not useless since there are cases when you just need the extra damage to ensure the kill (like boss-fights, high tier mobs with enormous hp pool), but it is advisable to set the still to be not used automatically in the AI and only us it manually when needed via shortcut.
yes, you cant turn it off in the AI, then Sera will not use the legion, unless you specifically order her to, that way she will have chance to attack with her sting and venom, and score you the kill.