Shen's ult saving backs?

So I was playing Shen last game and failed horribly except one bizarre moment, My allied MF was backing and I see Kayn coming at her and throw my ult on her right as he comes into the bush she is backing in, then Kayn goes ahead and gives her a whack... to no effect, the shield went down a little and the back finished right as he's winding up for another. So the thing I'm seeing is that she got hit a good second before the back animation finished and got a second hit right as it finished but she still backed despite being hit thanks to the shield from Shen's ult, is this intentional or a bug? [sg-lux-2]