Tristana ult bug (or broken mechanic)

I have recently been experiencing a bug where if I hit something with 's ultimate, that champ had a blue circle under them for (about 5 minutes, though it may have something to do with deaths) that allows me to see where that champ is as long as they are on screen. I have tested it, and I can see champs through fog of war, in brushes and invisible champs using this "exploit" (NOTE: This has only been tested against , so it could be his fault)