sivir and ali

This is intended. If two or more spells hit a target with a spell shield within at nearly the exact same time, the spell shield will absorb all the spells. In this case, using Alistar's Headbutt + Pulverize combo will result in both abilities hitting the target almost instantaneously, which means that Sivir will block both spells with her spell shield.

This is a bug that is impossible to fix. The Alistar combo happens so fast that the server /client communication can't keep up with it and spell shields end up absorbing both abilities. This isn't exclusive to Sivir or even League of Legends. Pretty much every game that has a "single" ability block/reflect works this way because the server and client can't communicate fast enough to remove the buff from just one of two abilities that happen almost simultaneously.