It's not a bug. All stun spells or attacks with a stun will not only stun the target(s), but will also put on 4 stun blocks.
This was a temporary fix emplemented by KI to help reduce the amount of chain stunners in the arena.
Chain Stunners are people who do 4v4 and sometimes 3v3 matches where at least 2 of them consantly stun the other team, making it impossible for them to do anything, thereby letting their teammates attack and buff with ease.
This was included in the new update. Apparently it is a temporary fix to deal with chain stunning. All the stun spells do this now. It is a little weird the first time you see it!