Sion Q aggro reset

I've played multiple games now where the blue buff's aggro will reset and its hp will regen to almost full while jungling as Sion. Coming out of my Q's stun it loses aggro on me for some reason. this doesn't happen every game, but its happened a few times and it's not fun; especially ranked. It could happen with other ones too, but I'm not trying to get it to happen.