Found a bug, Maw of Malmortius shield duration only lasts 3 seconds. The tooltip states that it's 5 seconds.
I tested this several times, Hexdrinker worked as intended (5 second duration), when I upgraded to Maw, the duration was only 3 seconds.
Steps to reproduce:
1. Buy Hexdrinker
2. Upgrade to Maw of Malmortius
3. Take enough magic damage to trigger the passive.
4. Notice that it lasts only 3 seconds. As opposed to 5.
League of Legends Community