[Gameplay] Hecarim not immune to CC during ultimate

Was standing beside two teammates as an Ashe arrow (ultimate) was coming towards us. As Hecarim, I ulted toward the arrow (and a couple enemies).

What should have happened: Arrow hits me as I'm ulting, I'm immune to CC so it does nothing, I land from my ult applying fear to the two enemies I land on and proceed with Ashe's arrow being wasted.

What actually happened: Arrow hits me as I'm ulting, I finish my dash from R, I land stunned, my two allies who were beside me when I pressed R were somehow stunned from the splash, and the fear from my ultimate wasn't applied to the two enemies I landed on.

I suppose it's possible there was something to do with ping or packet loss or something (i.e. my screen shows me ulting before the arrow hits but it actually hits before I ulted) but then I'm not so sure why my ultimate went on cooldown and I traveled the length of my ultimate before being stunned...