Sweeper Trinket, Teemo Shrooms, and Bushes

This was a while ago, but I remember playing a game against a Teemo and I couldn't sweeper any of his mushrooms that were in a bush without actually entering the bush, even if I had vision of it from a ward. This was especially frustrating because by entering the bush it meant you had to trigger the shroom. I'm not sure if this was meant of if it's a bug.