Smite cancelling movement/attack commands (specifically Hecarim running at someone with E active)

This is something I've only noticed recently, but while moving towards someone as Hecarim with E active, I will smite them as I approach and it will cancel my movement, which is very annoying for fairly obvious reasons. I'm not sure if it is only a specific set of conditions (Hecarim, with E active, challenging smite) and haven't done any testing but I play a lot of jungle and i feel like smite stopping movement is something irritating enough that I would have noticed it before if it happened every time...