Nasus Wither Cancels Auto-Attacks [GAMEPLAY]

I was just playing a game as Nautilus and in said game there was a Nasus on the enemy team. I would try and combo using Q, AA, W(for the auto reset), E, R. This Nasus however, could completely shut down my combo after the Q->AA just by withering me. As I was in the middle of attacking him with the reset auto from casting W, he'd wither me and it'd cancel my attack animation and make me start another animation with the slower attack speed. This caused a plenitude of unfortunate deaths and missed kills. I'm not entirely sure that this is a bug, but I haven't experienced it before and I can't find any information saying that this is normal. Therefore, I believe that it is a bug.