This is not a bug, it is intentional to reflect (and game wise "ungently" steer in the direction of) the Huns disappearing from the stage after Attila. The vassals historically took the opportunity to "throw off the Hunnic yoke" after Attila's death (even one of his most trusted companions) and in the game don't differentiate between Attila's heir and his brothers, they're all Huns to them, so if they declare war on White and Black Huns then it's because of that. And the neighbours declaring wars are interpreted as taking advantage of the chaos upon Attila's death. Having said this, do you by -5 stability mean that vassals and neighbours get it? That is not the idea, so then I will have to have a closer look and adjust it. But all in all, these events and penalties for the Huns are there to inverse the previous success prior to and during Attila.