heart seeker varus ult invisible when ulted from out of vision

i was playing a game as chogath when i suddenly became rooted in place for no visible reason then heart seeker varus stepped out from the fog of war and started autoing me and a couple seconds i could move again. after the skirmish was over i asked him if he ulted me he said he did. so the rest of his ults i could see just not this one that came from fog of war i didn't get video and i cant really test it since i dont own heart seeker varus idk if its just with that skin thou or all versions of varus. anyways hope it helps :D